[OpenMP] Remove implicit data sharing using device shared memory from libomptarget

Summary:
This patch reverts the changes to libomptarget that were coupled with the changes to Clang code gen for data sharing using shared memory. A similar patch exists for Clang: D43625

Shared memory is meant to be used as an optimization on top of a more general scheme. So far we didn't have a global memory implementation ready so shared memory was a solution which applied to the current level of OpenMP complexity supported by trunk on GPU devices (due to the missing NVPTX backend patch this functionality has never been exercised). Now that we have a global memory solution this patch is "in the way" and needs to be removed (for now). This patch (or an equivalent version of it) will be put out for review once the global memory scheme is in place.


Reviewers: ABataev, grokos, carlo.bertolli, caomhin

Reviewed By: grokos

Subscribers: Hahnfeld, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D43626

llvm-svn: 326950