[llvm-branch-commits] [clang-tools-extra] Drop option to use external shell (PR #209625)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/209625

The external shell is going to be removed from lit soon, so drop the
option to enable using it.