Remove xUnit from source build external packages

[mthalman]

The only dependency on xUnit from a source build perspective was the scenario-tests repo. With the changes in dotnet/sdk#48327, the scenario-tests repo can be built in an isolated manner with no dependency on source-build-externals. Therefore, the xunit submodule can be removed from source-build-externals.

[MichaelSimons]

Update: This work is no longer blocked.
This is blocked on #5061