Clean up HashSet.cs copy

We have a partial copy of HashSet as it existed in the .NET Framework circa 2008 or something, with the following comment:
https://github.com/dotnet/msbuild/blob/6a79376cc50e5f0d829475adc418e7078776ccb7/src/Build/Collections/RetrievableEntryHashSet/HashSet.cs#L19-L39

The rationale for this odd pattern of fragmented dead code was to diff for future updates to HashSet -- but the live copy of HashSet [here](https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs) has since changed so substantially that it can likely hardly be diffed at all.

As a matter of healthy cleanup, I suggest to delete all the dead code.

There have also been several improvements to HashSet since the copy was done, for perf reasons - either obvious ones or to produce better codegen. It would be nice to copy them (again, without bringing dead code) for some small perf wins. I expect this will be fairly easy to do by eyeballing them side by side.