Revert "Fix missing build dependency on omp_gen."

This reverts commit ed4328c6.

My change apparently caused a buildbot to fail with the error

  CMake Error at /b/sanitizer-x86_64-linux-autoconf/build/tsan_release_build/lib/cmake/llvm/AddLLVM.cmake:869 (add_dependencies):
  The dependency target "omp_gen" of target "ScudoBenchmarks.x86_64" does not
  exist.

I don't at all understand why, because as far as I can see, the target
`omp_gen` is only added to `LLVM_COMMON_DEPENDS` after having been
created, so there //should// be no way it can end up on anything's
dependency list if it doesn't exist! But apparently it happened anyway.

Differential Revision: https://reviews.llvm.org/D82659