[lldb][cmake] Remove custom logic for finding VCS file to fix LLDB's VCSVersion.inc generation

We are still implementing our own logic for this that looks for a VCS file in
the place where it was before the monorepo migration. This removes this logic
and just uses the CMake function that LLVM/Clang are using.

Reviewed By: JDevlieghere, kastiglione

Differential Revision: https://reviews.llvm.org/D88950