runtime: turn off checkmark by default

Change-Id: Ic8cb8b1ed8715d6d5a53ec3cac385c0e93883514
Reviewed-on: https://go-review.googlesource.com/9825Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/runtime1.go

@@ -336,7 +336,6 @@ var dbgvars = []dbgVar{
 }
 
 func parsedebugvars() {
-	debug.gccheckmark=1
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""
 		i := index(p, ",")