Remove obsolete git mirror files (dotnet/coreclr#27584)

Commit migrated from https://github.com/dotnet/coreclr/commit/db5f07cc6a51a8cb3d0b59a53f4313f086da2765

docs/coreclr/.gitmirrorall

-This folder will be mirrored by the Git-TFS Mirror recursively.
\ No newline at end of file

src/coreclr/.gitmirrorselective

-src
\ No newline at end of file

src/coreclr/cross/.gitmirrorall

-This folder will be mirrored by the Git-TFS Mirror recursively.
\ No newline at end of file

src/coreclr/src/.gitmirrorselective

-jit
-ToolBox
\ No newline at end of file

src/coreclr/src/ToolBox/.gitmirrorselective

-superpmi
\ No newline at end of file

src/coreclr/tests/.gitmirrorall

-This folder will be mirrored by the Git-TFS Mirror recursively.
\ No newline at end of file