ofproto: Remove ofproto_group_write_lookup()

ofproto_group_write_lookup() slightly different from
ofproto_group_lookup() in handling reference counting.
Currently, it has only one caller: modify_group().
It seems the abstraction is not adding value here.

Remove the function, along with some refactoring, makes modify_group()
easier to understand.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ryan Wilson <wryan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif: remove a redundant assignment

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif-xlate: Simplify group_is_alive

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif-xlate: Coding style fix for bucket_is_alive()

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>