runtime: remove unused file mkversion.c

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/10107044

src/pkg/runtime/mkversion.c

-// 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.
-
-// +build ignore
-
-#include <u.h>
-#include <libc.h>
-
-char *template =
-	"// AUTO-GENERATED by autogen.sh; DO NOT EDIT\n\n"
-	"package runtime\n"
-	"const defaultGoroot = `%s`\n"
-	"const theVersion = \"%s\"\n";
-
-void
-main(void)
-{
-	print(template, getgoroot(), getgoversion());
-	exits(0);
-}