btrfs: scrub: remove btrfs_fs_info::scrub_wr_completion_workers

Since the scrub rework introduced by commit 2af2aaf9 ("btrfs:
scrub: introduce structure for new BTRFS_STRIPE_LEN based interface")
and later commits, scrub only needs one single workqueue,
fs_info::scrub_worker.

That scrub_wr_completion_workers is initially to handle the delay work
after write bios finished.  But the new scrub code goes submit-and-wait
for write bios, thus all the work are done inside the scrub_worker.

The last user of fs_info::scrub_wr_completion_workers is removed in
commit 16f93993

 ("btrfs: scrub: remove the old writeback
infrastructure"), so we can safely remove the workqueue.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>