Remove DOTNET_HOST_PATH workaround when updated images are available

[333fred]

Versions of msbuild prior to 17.14.23+b0019275e will not set DOTNET_HOST_PATH, which breaks our ability to have a local SDK. As of this bug, 3 legs are affected by this:

• roslyn-CI (Correctness Correctness_Build_Artifacts)
• roslyn-integration-CI (Windows_Debug_Build Build_Windows_Debug)
• roslyn-integration-CI (Windows_Release_Build Build_Windows_Release)

When these legs are updated to a new image with at least that version of MSBuild installed, the workarounds tracked in code by a link to this comment can be removed.