runtime: leave gccheckmark testing off by default

It's not helping anymore, and it's fooling people who try to
understand performance (like me).

Change-Id: I133a644acae0ddf1bfa17c654cdc01e2089da963
Reviewed-on: https://go-review.googlesource.com/9018Reviewed-by: Austin Clements <austin@google.com>

src/runtime/runtime1.go

@@ -334,9 +334,6 @@ var dbgvars = []dbgVar{
 }
 
 func parsedebugvars() {
-	// gccheckmark is enabled by default for the 1.5 dev cycle
-	debug.gccheckmark = 1
-
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""
 		i := index(p, ",")