libgomp GCN plugin: Clean up always-empty 'PLUGIN_GCN_CPPFLAGS', 'PLUGIN_GCN_LDFLAGS'

libgomp GCN plugin: Clean up always-empty 'PLUGIN_GCN_CPPFLAGS', 'PLUGIN_GCN_LDFLAGS'

After recent commit d6adba307508c75f1ccb2121eb1a43c9ab1d4056
"libgomp GCN plugin: Clean up unused references to system-provided HSA Runtime
library", these aren't set anymore.

	libgomp/
	* plugin/Makefrag.am (libgomp_plugin_gcn_la_CPPFLAGS): Don't
	consider 'PLUGIN_GCN_CPPFLAGS'.
	(libgomp_plugin_gcn_la_LDFLAGS): Don't consider
	'PLUGIN_GCN_LDFLAGS'.
	* plugin/configfrag.ac (PLUGIN_GCN_CPPFLAGS, PLUGIN_GCN_LDFLAGS):
	Remove.
	* Makefile.in: Regenerate.
	* configure: Likewise.
	* testsuite/Makefile.in: Likewise.