runtime/cgo: remove unused variable

I missed this one in golang.org/cl/2232 and only tested the patch
on openbsd/amd64.

Change-Id: I4ff437ae0bfc61c989896c01904b6d33f9bdf0ec
Reviewed-on: https://go-review.googlesource.com/2234
Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/cgo/gcc_openbsd_386.c

@@ -114,7 +114,6 @@ x_cgo_init(G *g, void (*setg)(void*))
 {
 	pthread_attr_t attr;
 	size_t size;
-	void *handle;
 
 	setg_gcc = setg;
 	pthread_attr_init(&attr);