runtime: remove guard against uninitialized forcegc.g

The race was in the old C code.
The new Go code does not have the race
and does not need the check.

LGTM=bradfitz, dvyukov
R=golang-codereviews, bradfitz, dvyukov
CC=golang-codereviews, rlh
https://golang.org/cl/140180043

src/pkg/runtime/proc.c

@@ -2955,12 +2955,9 @@ sysmon(void)
 		lastgc = runtime·atomicload64(&mstats.last_gc);
 		if(lastgc != 0 && unixnow - lastgc > forcegcperiod && runtime·atomicload(&runtime·forcegc.idle)) {
 			runtime·lock(&runtime·forcegc.lock);
-			if(runtime·forcegc.g != nil) {
-				// Goroutine may be started but has not initialized g yet.
-				runtime·forcegc.idle = 0;
-				runtime·forcegc.g->schedlink = nil;
-				injectglist(runtime·forcegc.g);
-			}
+			runtime·forcegc.idle = 0;
+			runtime·forcegc.g->schedlink = nil;
+			injectglist(runtime·forcegc.g);
 			runtime·unlock(&runtime·forcegc.lock);
 		}