Fix incremental build issue with package files and doc file

The doc file for a project is not re-generated unless there are new APIs. This means it can be older than a package file (i.e. a .targets file) in the project, and remain so for a long time across many builds.

This slows down builds unnecessarily, so remove. If package really needs a new build, you can always rebuild before Pack.

Author: kzu

src/Directory.Build.targets

@@ -96,7 +96,7 @@
 
   <ItemGroup>
     <!-- Consider the project out of date if any of these files changes -->
-    <UpToDateCheck Include="@(PackageFile);@(None);@(Content);@(EmbeddedResource)" />
+    <UpToDateCheck Include="@(None);@(Content);@(EmbeddedResource)" />
     <!-- We'll typically use ThisAssembly.Strings instead of the built-in resource manager codegen -->
     <EmbeddedResource Update="@(EmbeddedResource)"  Generator="" Condition="'$(EnableRexCodeGenerator)' != 'true'" />
   </ItemGroup>