runtime: assert p.destroy runs with GC not running

This is already guaranteed by stopTheWorldGC prior to procresize. Thus
the cleanup code here is dead, which is a bit confusing.

Replace it with a throw for clarity.

Change-Id: I6a6a636c8ca1487b720c4fab41b2b86c13d1d9e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/709655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: prattmic

src/runtime/proc.go

@@ -5818,11 +5818,13 @@ func (pp *p) destroy() {
 	// Move all timers to the local P.
 	getg().m.p.ptr().timers.take(&pp.timers)
 
-	// Flush p's write barrier buffer.
-	if gcphase != _GCoff {
-		wbBufFlush1(pp)
-		pp.gcw.dispose()
+	// No need to flush p's write barrier buffer or span queue, as Ps
+	// cannot be destroyed during the mark phase.
+	if phase := gcphase; phase != _GCoff {
+		println("runtime: p id", pp.id, "destroyed during GC phase", phase)
+		throw("P destroyed while GC is running")
 	}
+
 	clear(pp.sudogbuf[:])
 	pp.sudogcache = pp.sudogbuf[:0]
 	pp.pinnerCache = nil