loop: Remove struct loop_func_table

The struct is introduced in the commit 754d9679
("loop: remove loop.h"), but it is not used now.
So remove it.

No functional changes.

Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250227163343.55952-1-yanjun.zhu@linux.dev


Signed-off-by: Jens Axboe <axboe@kernel.dk>