Revert "[clang] [unittest] Add a test for Generic_GCC::GCCVersion::Parse (#69078)"

This reverts commit b4b35a5d.

That commit broke builds with -DBUILD_SHARED_LIBS=ON. The reason
is that clang::driver::toolchains::Generic_GCC::GCCVersion::Parse
isn't visible outside of the shared library, because
the Generic_GCC class is marked with LLVM_LIBRARY_VISIBILITY.