ext4: remove obsolete comment from ext4_can_extents_be_merged()

Support for unwritten extents was added to ext4 a long time ago, so
remove a misleading comment that says they're a future feature.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191231180444.46586-7-ebiggers@kernel.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>