btrfs: remove old unused commented out code

Remove code which has been #if0-ed out for a very long time and does not
seem to be related to current codebase anymore.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/delayed-ref.c
Simple merge

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/extent-tree.c
Simple merge

diff --cc fs/btrfs/inode.c
Simple merge

diff --cc fs/btrfs/transaction.c
Simple merge