Cleanup: Delete NGen of T

[ladipro]

Context

The sole purpose of introducing the type seems to have been silencing a legacy code analyzer rule. The rule does not exist anymore / has not been brought over to Roslyn (dotnet/roslyn-analyzers#722) and it's now hurting performance, if anything. Types like HashSet<int> are part of the mscorlib native image and it's wasteful to duplicate the code in our binaries. The rest is handled by IBC/OptProf.

Changes Made

Deleted NGen and its uses.

Testing

Experimental insertion to confirm no regressions.

[rainersigwald]

For these cleanups of stale .NET Framework optimization tricks, can we wait to put them in until 17.9 codeflow is unblocked and we can get a good run with optprof to just validate that they're actually useless as we expect?

[ladipro]

Sure, this can wait. For what it's worth, there is no impact on 17.8 PerfDDRITs (tested both this change and the removal of Pair<TKey, TValue>).

[rainersigwald]

For these cleanups of stale .NET Framework optimization tricks, can we wait to put them in until 17.9 codeflow is unblocked and we can get a good run with optprof to just validate that they're actually useless as we expect?

We should be in good shape now.

[ladipro]

@JaynieBai thank you for resolving the conflicts! I'll watch VS insertions to confirm no impact on NGEN-related metrics.

[ladipro]

No regressions in the experimental insertion. This is good to go.