ext4: remove unused macro MPAGE_DA_EXTENT_TAIL

Remove unused macro MPAGE_DA_EXTENT_TAIL which
is no more used after below commit
4e7ea81d ("ext4: restructure writeback path")

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200101095137.25656-1-riteshh@linux.ibm.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>