[GCN] Streamline 'libgomp/testsuite/lib/libgomp.exp:check_effective_target_openacc_ra...

[GCN] Streamline 'libgomp/testsuite/lib/libgomp.exp:check_effective_target_openacc_radeon_accel_selected'

The GCN support that got added in r278935 (commit
83caa34e2a618842e05f59cbb3e2dda93dc23270) "Enable OpenACC GCN testing" was
forked before my r269107 (commit ee332b4a9a19552d160a23155f59b11692d8f07e)
"[libgomp] Clarify difference between offload target, offload plugin, and
OpenACC device type", and didn't later pick up these changes.

No functional change.

	libgomp/
	* testsuite/lib/libgomp.exp
	(check_effective_target_openacc_radeon_accel_selected):
	Streamline.