lightnvm: pblk: remove pblk_gc_stop

pblk_gc_stop just sets pblk->gc->gc_active to zero, ignoring
the flush parameter. This is plain confusing, so remove the
function and set the gc active flag at the call points instead.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>