remove dead code from nativemethods

[kasperk81]

followup on #9223

[JanKrivanek]

Hello @kasperk81 - thank you for your contribution! MSBuild team for sure appraciates contributions. Upfront heads up via bug or other form of discussion is preferred (plus an explicit confirmation of wanting to fix).

More details: https://github.com/dotnet/msbuild/blob/main/documentation/wiki/Contributing-Code.md

Each PR should ideally have motivation and goal in the description (links are big advantage, but important context should be part of the PR).

By no means would I want to kill the interest and flow :-) But it should be just little extra work that will save us considerable time.

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In this specific PR - it mentiones it's a followup of the other PR that uses IO.Redist on NetFx, while here only single implementation is used - it's bit confusing to me. It looks great as code cleanup. Though as mentioned followup I'd expect the usage of IO.Redist or justification why not using it

[kasperk81]

while here only single implementation is used

the change in WindowsFileSystem.cs is following the other pr, MSBuildTaskHostFileSystem.cs is only built on netfx so it didn't required netcore tfm branch and that's the only difference there.