Ensure 6.0 SDK diff disappears: `sdk/6.0.100/Sdks/Microsoft.NET.Sdk/tools/net472/Microsoft.NET.Build.Tasks.dll`

[dagood]

This file is necessary for OmniSharp and should be back after dotnet/installer#12497. We probably just need to run a new build and refresh the diff to make sure this file reappears in the final SDK after that PR went in.

[dagood]

@crummel and @eerhardt confirmed that OmniSharp works in a recent build, and I confirmed this diff is gone in my build.

Follow-up:

• Add OmniSharp smoke-test #1950