openacc: Revert erroneous gang reduction changes

openacc: Revert erroneous gang reduction changes

This patch reverts some changes related to "gang reduction on an orphan loop"
of commit 3a5e525489f2f808093ae1f12b5d2b406f571ec7 "Various OpenACC reduction
enhancements - FE change" similar to the mainline commit
77d24d43644909852998043335b5a0e09d1e8f02.

gcc/c/ChangeLog:

	* c-typeck.cc (c_finish_omp_clauses): Remove "gang reduction on an
	orphan loop" checking.

gcc/cp/ChangeLog:

	* semantics.cc (finish_omp_clauses): Remove "gang reduction on an
	orphan loop" checking.

gcc/fortran/ChangeLog:

	* openmp.cc (oacc_is_parallel): Remove.
	(resolve_oacc_loop_blocks): Remove "gang reduction on an orphan loop"
	checking.