Currently we harvestwbufs the moment we enter the mark phase, even
before starting the world again. Since cached wbufs are only filled
when we're in mark or mark termination, they should all be empty at
this point, making the harvest pointless. Remove the harvest.

We should, but do not currently harvest at the end of the mark phase
when we're running out of work to do.

Change-Id: I5f4ba874f14dd915b8dfbc4ee5bb526eecc2c0b4
Reviewed-on: https://go-review.googlesource.com/7669Reviewed-by: Rick Hudson <rlh@golang.org>