[OpenMP][CMake] Revert standalone build LIBOMP_HEADERS_INSTALL_PATH (#91243)

Revert the portion of https://github.com/llvm/llvm-project/pull/75125
which modified the LIBOMP_HEADERS_INSTALL_PATH in standalone build.

This change is harmful for real standalone builds (i.e. builds where we
build openmp by itself), since it tries to overwrite the `omp.h` inside
the build compiler. For all-in-one builds with clang, testing shows this
change is unnecessary as https://github.com/llvm/llvm-project/pull/88007
already set up that build configuration so that omp.h will be put into
the project build's `clang` resource directory.