[libomptarget][NVPTX] Drop dead code and data structures, NFCI.

 * cg and HasCancel in WorkDescr were never read and can be removed.
 * This eliminates the last use of priv in ThreadPrivateContext.
 * CounterGroup is unused afterwards.
 * Remove duplicate external declares in omptarget-nvptx.cu that are
   already in the header omptarget-nvptx.h.

Differential Revision: https://reviews.llvm.org/D51622

llvm-svn: 341370