runtime: don't disable GC work caching during mark termination

Currently, we disable GC work caching during mark termination. This is
no longer necessary with the new mark completion detection because

1. There's no way for any of the GC mark termination helpers to have
any real work queued and,

2. Mark termination has to explicitly flush every P's buffers anyway
in order to flush Ps that didn't run a GC mark termination helper.

Hence, remove the code that disposes gcWork buffers during mark
termination.

Updates #26903. This is a follow-up to eliminating mark 2.

Change-Id: I81f002ee25d5c10f42afd39767774636519007f9
Reviewed-on: https://go-review.googlesource.com/c/134320
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

Author: aclements

src/runtime/mgc.go

@@ -1931,7 +1931,6 @@ func gcMark(start_time int64) {
 	} else {
 		gcDrain(gcw, gcDrainNoBlock)
 	}
-	gcw.dispose()
 
 	if debug.gccheckmark > 0 {
 		// This is expensive when there's a large number of
@@ -2130,7 +2129,6 @@ func gchelper() {
 		} else {
 			gcDrain(gcw, gcDrainNoBlock)
 		}
-		gcw.dispose()
 	}
 
 	nproc := atomic.Load(&work.nproc) // work.nproc can change right after we increment work.ndone

src/runtime/mgcmark.go

@@ -1149,11 +1149,6 @@ func shade(b uintptr) {
 	if obj, span, objIndex := findObject(b, 0, 0); obj != 0 {
 		gcw := &getg().m.p.ptr().gcw
 		greyobject(obj, 0, 0, span, gcw, objIndex)
-		if gcphase == _GCmarktermination {
-			// Ps aren't allowed to cache work during mark
-			// termination.
-			gcw.dispose()
-		}
 	}
 }
 

src/runtime/mwbbuf.go

@@ -275,9 +275,4 @@ func wbBufFlush1(_p_ *p) {
 
 	// Enqueue the greyed objects.
 	gcw.putBatch(ptrs[:pos])
-	if gcphase == _GCmarktermination {
-		// Ps aren't allowed to cache work during mark
-		// termination.
-		gcw.dispose()
-	}
 }