libgomp GCN plugin: Clean up unused references to system-provided HSA Runtime library

libgomp GCN plugin: Clean up unused references to system-provided HSA Runtime library

This is only active if GCC is 'configure'd with '--with-hsa-runtime=[...]' or
'--with-hsa-runtime-include=[...]', '--with-hsa-runtime-lib=[...]' -- which
nobody really is doing, as far as I can tell.

Originally changed for the libgomp HSA plugin in
commit b8d89b03db5f212919e4571671ebb4f5f8b1e19d (r242749)
"Remove build dependence on HSA run-time", and later propagated into the GCN
plugin, these are no longer built against system-provided HSA Runtime library.
Instead, unconditionally built against the GCC-shipped 'include/hsa*.h' header
files, and at run time does 'dlopen("libhsa-runtime64.so.1")'.  It thus doesn't
make sense to consider references to system-provided HSA Runtime library during
libgomp GCN plugin build.

	libgomp/
	* plugin/configfrag.ac (HSA_RUNTIME_CPPFLAGS)
	(HSA_RUNTIME_LDFLAGS): Remove.
	* configure: Regenerate.