qcow2: Remove BDS parameter from qcow2_cache_discard()

qcow2: Remove BDS parameter from qcow2_cache_discard()

This function was only using the BlockDriverState parameter to pass it
to qcow2_cache_get_table_idx() and qcow2_cache_table_release(). This
is no longer necessary so this parameter can be removed.

Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 9724f7e38e763ad3be32627c6b7fe8df9edb1476.1517840877[email protected]
Signed-off-by: Max Reitz <[email protected]>