block/throttle: Remove protocol-related fields

The throttle driver is not a protocol so it should implement bdrv_open
instead of bdrv_file_open and not provide a protocol_name.

Attempts to invoke this driver using protocol syntax
(i.e. throttle:<filename:options:...>) will now fail gracefully:

  $ qemu-img info throttle:foo
  qemu-img: Could not open 'throttle:foo': Unknown protocol 'throttle'
Signed-off-by: NFabiano Rosas <farosas@linux.vnet.ibm.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/throttle.c

@@ -215,10 +215,9 @@ static void coroutine_fn throttle_co_drain_end(BlockDriverState *bs)
 
 static BlockDriver bdrv_throttle = {
     .format_name                        =   "throttle",
-    .protocol_name                      =   "throttle",
     .instance_size                      =   sizeof(ThrottleGroupMember),
 
-    .bdrv_file_open                     =   throttle_open,
+    .bdrv_open                          =   throttle_open,
     .bdrv_close                         =   throttle_close,
     .bdrv_co_flush                      =   throttle_co_flush,