block: Remove bdrv_reset_dirty

block: Remove bdrv_reset_dirty

Using this function would always be wrong because a dirty bitmap must
have a specific owner that consumes the dirty bits and calls
bdrv_reset_dirty_bitmap().

Remove the unused function to avoid future misuse.

Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: John Snow <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>