Revert "[Flang] [FlangRT] Introduce FlangRT project as solution to Flang's runtime LLVM integration"

This reverts commit 6403287eff71a3d6f6c862346d6ed3f0f000eb70.

This is failing on all but 1 of Linaro's flang builders.
CMake Error at /home/tcwg-buildbot/worker/clang-aarch64-full-2stage/llvm/flang-rt/unittests/CMakeLists.txt:37 (message):
  Target llvm_gtest not found.

GitOrigin-RevId: ffc67bb3602a6a9a4f886af362e1f2d7c9821570