[build] Don't set $(ImplicitlyExpandDesignTimeFacades)=False

Supporting both OS X and Linux is "interesting", for not fully
understood reasons.

On the one hand, the OS X `src/Mono.Posix` build was failing because
there were *too many* references to `System.Runtime` (9a50c66):

	CSC: error CS1703: An assembly `System.Runtime' with the same identity
	has already been imported. Consider removing one of the references.

Meanwhile, after commit 9a50c66 @atsushieno is unable to build
`src/Mono.Posix` on Linux:

	error CS0012: The type `System.IDisposable' is defined in an assembly
	that is not referenced. Consider adding a reference to assembly
	`System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

The current hypothesis is that setting
`$(ImplicitlyExpandDesignTimeFacades)` to False [0] is responsible for
this, and the fix is to:

1. Don't Do That™: don't override
    `$(ImplicitlyExpandDesignTimeFacades)`.

2. Remove the explicit reference to `System.Runtime` in those
    projects.

Let's see if that works.

[0]: Related question: *why* was
    `$(ImplicitlyExpandDesignTimeFacades)` overridden in the first
    place? It was overridden so that we'd use the Xamarin.Android
    copy of `System.Runtime.dll` instead of the "system" one.
    However, this shouldn't be necessary, as we're not providing a
    custom PCL profile, so the normal `System.Runtime.dll` is fine.

Author: jonpryor

src/Mono.Android.Export/Mono.Android.Export.csproj

@@ -52,10 +52,6 @@
       <HintPath>$(OutputPath)..\v1.0\Java.Interop.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Runtime">
-      <HintPath>$(OutputPath)..\v1.0\Facades\System.Runtime.dll</HintPath>
-      <Private>False</Private>
-    </Reference>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Properties\AssemblyInfo.cs" />
@@ -110,9 +106,6 @@
     <Compile Include="Mono.CodeGeneration\Exp.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
-  <PropertyGroup>
-    <ImplicitlyExpandDesignTimeFacades>False</ImplicitlyExpandDesignTimeFacades>
-  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\Mono.Android\Mono.Android.csproj">
       <Project>{66CF299A-CE95-4131-BCD8-DB66E30C4BF7}</Project>

src/Mono.Android/Mono.Android.csproj

@@ -63,10 +63,6 @@
       <HintPath>$(OutputPath)..\v1.0\Java.Interop.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Runtime">
-      <HintPath>$(OutputPath)..\v1.0\Facades\System.Runtime.dll</HintPath>
-      <Private>False</Private>
-    </Reference>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Android\IncludeAndroidResourcesFromAttribute.cs" />
@@ -302,9 +298,6 @@
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="Mono.Android.targets" />
   <Import Project="$(IntermediateOutputPath)android-$(AndroidApiLevel)\Mono.Android.projitems" Condition="Exists('$(IntermediateOutputPath)android-$(AndroidApiLevel)\Mono.Android.projitems')" />
-  <PropertyGroup>
-    <ImplicitlyExpandDesignTimeFacades>False</ImplicitlyExpandDesignTimeFacades>
-  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\build-tools\api-merge\api-merge.csproj">
       <Project>{3FC3E78B-F7D4-42EA-BBE8-4535DF42BFF8}</Project>

src/Xamarin.Android.NUnitLite/Xamarin.Android.NUnitLite.csproj

@@ -58,10 +58,6 @@
       <HintPath>$(OutputPath)..\v1.0\Java.Interop.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Runtime">
-      <HintPath>$(OutputPath)..\v1.0\Facades\System.Runtime.dll</HintPath>
-      <Private>False</Private>
-    </Reference>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Resources\Resource.designer.cs" />
@@ -348,9 +344,6 @@
     <AndroidResource Include="Resources\layout\test_suite.axml" />
   </ItemGroup>
   <Import Project="$(OutputPath)\..\..\..\xbuild\Xamarin\Android\Xamarin.Android.CSharp.targets" />
-  <PropertyGroup>
-    <ImplicitlyExpandDesignTimeFacades>False</ImplicitlyExpandDesignTimeFacades>
-  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\Mono.Android\Mono.Android.csproj">
       <Project>{66CF299A-CE95-4131-BCD8-DB66E30C4BF7}</Project>