io_uring/kbuf: remove dead define

We no longer use IO_BUFFER_LIST_BUF_PER_PAGE, kill it.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -17,8 +17,6 @@
 #include "kbuf.h"
 #include "memmap.h"
 
-#define IO_BUFFER_LIST_BUF_PER_PAGE (PAGE_SIZE / sizeof(struct io_uring_buf))
-
 /* BIDs are addressed by a 16-bit field in a CQE */
 #define MAX_BIDS_PER_BGID (1 << 16)