Revert D118493 "Set rpath on openmp executables"

This reverts commit 9b9d0811.
(Accepted by Jon https://reviews.llvm.org/D118493#4178250)

libc++, libc++abi, libunwind, and compiler-rt don't add the extra DT_RUNPATH,
it's strange for OpenMP to diverge.
Some build systems want to handle DT_RUNPATH themselves (e.g.
CMAKE_INSTALL_RPATH). Some distributions (e.g. Fedora) have policies against
DT_RUNPATH and the default DT_RUNPATH for OpenMP is causing trouble.

For users who don't want to specify rpath by themselves,
https://clang.llvm.org/docs/UsersManual.html#configuration-files
can be used to specify the default rpath, e.g.
specify -frtlib-add-rpath or -Wl,-rpath in bin/clang.cfg