io_uring: Drop per-ctx dummy_ubuf

Commit 19a63c40 ("io_uring/rsrc: keep one global dummy_ubuf")
replaced it with a global static object but this stayed behind.

Fixes: 19a63c40 ("io_uring/rsrc: keep one global dummy_ubuf")
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20240523214517.31803-1-krisman@suse.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -373,7 +373,6 @@ struct io_ring_ctx {
 	struct io_restriction		restrictions;
 
 	/* slow path rsrc auxilary data, used by update/register */
-	struct io_mapped_ubuf		*dummy_ubuf;
 	struct io_rsrc_data		*file_data;
 	struct io_rsrc_data		*buf_data;