ext4: remove redundant check for first_not_zeroed in ext4_register_li_request

We have checked first_not_zeroed == ngroups already above, so remove
this redundant check.

sbi->s_li_request = NULL above is also removed since it is NULL
already.

Cc: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>