ext4: delete declaration for ext4_split_extent()

There are no forward references for ext4_split_extent() in extents.c,
so delete its unnecessary declaration.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20200212162141.22381-1-enwlinux@gmail.com


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