[libomptarget-nvptx] Remove dead functions

These entry points are never called by Clang trunk nor clang-ykt. If
XL doesn't use them either, they can finally go away.

Differential Revision: https://reviews.llvm.org/D52700

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@365817 91177308-0d34-0410-b5e6-96231b3b80d8