block: drop dead assignments in loop_init()

Commit 8410d38c ("loop: use __register_blkdev to allocate devices on
demand") simplified loop_init(); so computing the range of the block region
is not required anymore and can be dropped.

Drop dead assignments in loop_init().

As compilers will detect these unneeded assignments and optimize this,
the resulting object code is identical before and after this change.

No functional change. No change in object code.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>