runtime: remove GC start up/shutdown workaround in mallocgc

Currently mallocgc detects if the GC is in a state where it can't
assist, but also can't allocate uncontrolled and yields to help out
the GC. This was a workaround for periods when we were trying to
schedule the GC coordinator. It is no longer necessary because there
is no GC coordinator and malloc can always assist with any GC
transitions that are necessary.

Updates #11970.

Change-Id: I4f7beb7013e85e50ae99a3a8b0bb708ba49cbcd4
Reviewed-on: https://go-review.googlesource.com/16392Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/malloc.go

@@ -736,17 +736,6 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 
 	if shouldhelpgc && gcShouldStart(false) {
 		gcStart(gcBackgroundMode, false)
-	} else if shouldhelpgc && bggc.working != 0 && gcBlackenEnabled == 0 {
-		// The GC shutting down, so we can't
-		// assist, but we also can't allocate unabated. Slow
-		// down this G's allocation and help the GC stay
-		// scheduled by yielding.
-		//
-		// TODO: This is unused. Remove.
-		gp := getg()
-		if gp != gp.m.g0 && gp.m.locks == 0 && gp.m.preemptoff == "" {
-			Gosched()
-		}
 	}
 
 	return x