check.mak: for some weird reason valgrind does not report actual memleaks if GST_PLUGIN_PATH is set to anything but t...

Original commit message from CVS:

* check.mak:
for some weird reason valgrind does not report actual memleaks
if GST_PLUGIN_PATH is set to anything but the core gstreamer dir           while valgrind is running.  Since the registry is going to go
anyway, I don't want to waste any more time on this; I just run
valgrind without GST_PLUGIN_PATH set.  Since the registry loading
doesn't check if GST_PLUGIN_PATH got changed as a reason to rebuild
the registry, that's actually fine.