btrfs: don't needlessly create extent-refs kernel thread

btrfs: don't needlessly create extent-refs kernel thread

commit 80ed4548d0711d15ca51be5dee0ff813051cfc90 upstream.

The patch 32b593bfcb58 ("Btrfs: remove no longer used function to run
delayed refs asynchronously") removed the async delayed refs but the
thread has been created, without any use. Remove it to avoid resource
consumption.

Fixes: 32b593bfcb58 ("Btrfs: remove no longer used function to run delayed refs asynchronously")
CC: stable@vger.kernel.org # 5.2+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>