runtime: actually remove go-alloc.h

Accidentally omitted from the commit of CL 34797 (SVN revision 244036).

From-SVN: r244166

libgo/runtime/go-alloc.h

-/* go-alloc.h -- allocate memory for Go.
-
-   Copyright 2009 The Go Authors. All rights reserved.
-   Use of this source code is governed by a BSD-style
-   license that can be found in the LICENSE file.  */
-
-#include <stddef.h>
-#include <stdint.h>
-
-extern void *__go_alloc (unsigned int __attribute__ ((mode (pointer))));
-extern void __go_free (void *);