btrfs: Cleanup the old btrfs_worker.

Since all the btrfs_worker is replaced with the newly created
btrfs_workqueue, the old codes can be easily remove.

Signed-off-by: Quwenruo <[email protected]>
Tested-by: David Sterba <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>
5 files changed