loop: Remove struct loop_func_table

The struct is introduced in the commit 754d96798f
("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>

drivers/block/loop.c

@@ -45,8 +45,6 @@ enum {
 	Lo_deleting,
 };
 
-struct loop_func_table;
-
 struct loop_device {
 	int		lo_number;
 	loff_t		lo_offset;