[trace] Don't use pthread_key_delete.

On OpenBSD libc doesn't provide pthread stubs like glibc on Linux
or newer libcs from FreeBSD.  However libX11 does provide a stubs
for a subset of the pthread functions (formerly in libXThrStub,
now moved into libX11 proper), but pthread_key_delete() is not
one of the stubbed ones.  So, on OpenBSD cairo's non-linking of
libpthread accidentally works as long as the xlib-surface is enabled,
which is nearly always the case.

This patch makes trace.c stand at the same precipice as cairo itself
by reverting to only a subset of the pthreads functions stubbed by
libX11.

util/cairo-trace/trace.c

@@ -485,7 +485,6 @@ _fini_trace (void)
 	}
     }
 
-    pthread_key_delete (counter_key);
     pthread_mutex_destroy (&Types.mutex);
 }