fuse: removed unused function fuse_uring_create() from header

Function fuse_uring_create() is used only from dev_uring.c and does not
need to be exposed in the header file.  Furthermore, it has the wrong
signature.

While there, also remove the 'struct fuse_ring' forward declaration.

Signed-off-by: Luis Henriques <luis@igalia.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>