[llvm-branch-commits] [compiler-rt] Drop support for using the external shell (PR #209626)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/209626

The lit external shell is going to be removed soon, so drop support for
using it to run the compiler-rt tests.