ext4: remove dead GET_BLOCKS_ZERO code

There's no call to ext4_map_blocks() in the current ext4 code with a
flags argument that combines EXT4_GET_BLOCKS_CONVERT and
EXT4_GET_BLOCKS_ZERO.  Remove the code that corresponds to this case
from ext4_ext_handle_unwritten_extents().

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200430185320.23001-2-enwlinux@gmail.com


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