Revert "[libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (#85756)"

This reverts commit 272d1b44,
and the follow-up fix in d209d134.

Even after the follow-up fix, building with an empty
CMAKE_INSTALL_PREFIX errors out with errors like this:

    CMake Error at /b/s/w/ir/x/w/llvm-llvm-project/libcxx/modules/CMakeLists.txt:215 (file):
      file RELATIVE_PATH must be passed a full path to the directory:
      lib/x86_64-pc-windows-msvc