runtime: remove unused marking parfor

The GC now handles the root marking jobs as part of general marking,
so work.markfor is no longer used.

Change-Id: I6c3b23fed27e4e7ea6430d6ca7ba25ae4d04ed14
Reviewed-on: https://go-review.googlesource.com/16811
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgc.go

@@ -161,7 +161,6 @@ func gcinit() {
 		throw("size of Workbuf is suboptimal")
 	}
 
-	work.markfor = parforalloc(_MaxGcproc)
 	_ = setGCPercent(readgogc())
 	for datap := &firstmoduledata; datap != nil; datap = datap.next {
 		datap.gcdatamask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcdata)), datap.edata-datap.data)
@@ -743,7 +742,6 @@ var work struct {
 	nwait   uint32
 	ndone   uint32
 	alldone note
-	markfor *parfor
 
 	// Number of roots of various root types. Set by gcMarkRootPrepare.
 	nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int

src/runtime/mgcmark.go

@@ -97,7 +97,7 @@ var oneptrmask = [...]uint8{1}
 // Preemption must be disabled (because this uses a gcWork).
 //
 //go:nowritebarrier
-func markroot(desc *parfor, i uint32) {
+func markroot(i uint32) {
 	// TODO: Consider using getg().m.p.ptr().gcw.
 	var gcw gcWork
 
@@ -796,7 +796,7 @@ func gcDrain(gcw *gcWork, flags gcDrainFlags) {
 				break
 			}
 			// TODO: Pass in gcw.
-			markroot(nil, job)
+			markroot(job)
 		}
 	}