Reverted commits implementing OMPThreadPool.

clang and llvm versions used in cling do not support openmp. This would
break symmetry between what is available for user-compiled programs and
what is available in the interpreter. OS X clang version does not support
openmp yet either.

Commits reverted:
  o ee75770
    'fixed bugs in OMPThreadPool constructors'
  o a82731f
    'added an OMP-based implementation'