ext4: remove unneeded code in ext4_unlink

Setting retval to zero is not needed in ext4_unlink.
Remove unneeded code.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>