Btrfs: remove no longer used stuff for tracking pending ordered extents

Tracking pending ordered extents per transaction was introduced in commit
50d9aa99 ("Btrfs: make sure logged extents complete in the current
transaction V3") and later updated in commit 161c3549 ("Btrfs: change
how we wait for pending ordered extents").

However now that on fsync we always wait for ordered extents to complete
before logging, done in commit 5636cf7d ("btrfs: remove the logged
extents infrastructure"), we no longer need the stuff to track for pending
ordered extents, which was not completely removed in the mentioned commit.
So remove the remaining of the pending ordered extents infrastructure.

Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>