block: drop bdrv_set_key from BlockDriver

This is not used anymore since c01c214b ("block: remove all encryption
handling APIs", 2017-07-11).
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

include/block/block_int.h

@@ -127,7 +127,6 @@ struct BlockDriver {
                           Error **errp);
     void (*bdrv_close)(BlockDriverState *bs);
     int (*bdrv_create)(const char *filename, QemuOpts *opts, Error **errp);
-    int (*bdrv_set_key)(BlockDriverState *bs, const char *key);
     int (*bdrv_make_empty)(BlockDriverState *bs);
 
     void (*bdrv_refresh_filename)(BlockDriverState *bs, QDict *options);