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.de


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