openacc: Remove unnecessary detach finalization

openacc: Remove unnecessary detach finalization

The call to gomp_detach_pointer in gomp_unmap_vars_internal does not
need to force finalization, and doing so may mask mismatched pointer
attachments/detachments. This patch removes the forcing.

2020-07-16  Julian Brown  <julian@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>

libgomp/
	* target.c (gomp_unmap_vars_internal): Remove unnecessary forcing of
	finalization for detach operation.
	* testsuite/libgomp.oacc-c-c++-common/structured-detach-underflow.c:
	New test.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>