[OpenACC] Remove erroneous "Orphan reductions cannot have gang partitioning" handling

[OpenACC] Remove erroneous "Orphan reductions cannot have gang partitioning" handling

That is:

    -/* Ensure that the middle end does not assign gang level parallelism
    -   to orphan loop containing reductions.  */
    +/* Verify that we diagnose "gang reduction on an orphan loop" for automatically
    +   assigned gang level of parallelism.  */

... to implement what the OpenACC specification actually says.

Fix-up for preceding commit 2b7dac2c0dcb087da9e4018943c023c0678234a3
"Make OpenACC orphan gang reductions errors".

	gcc/
	* omp-offload.c (oacc_loop_auto_partitions): Remove erroneous
	"Orphan reductions cannot have gang partitioning" handling.
	gcc/testsuite/
	* c-c++-common/goacc/nested-reductions-1-routine.c: Adjust.
	* c-c++-common/goacc/nested-reductions-2-routine.c: Adjust.
	* c-c++-common/goacc/orphan-reductions-2.c: Adjust.
	* gfortran.dg/goacc/nested-reductions-1-routine.f90: Adjust.
	* gfortran.dg/goacc/nested-reductions-2-routine.f90: Adjust.
	* gfortran.dg/goacc/orphan-reductions-1.f90: Adjust.
	* gfortran.dg/goacc/orphan-reductions-2.f90: Adjust.