Ignore the return value from ack_keys.

Resolves:

riak_kv_index_fsm.erl:127: Expression produces a value of type {_,'ok'}, but this value is unmatched
riak_kv_index_fsm.erl:134: Expression produces a value of type {_,'ok'}, but this value is unmatched
riak_kv_index_fsm.erl:191: Expression produces a value of type {_,'ok'}, but this value is unmatched
riak_kv_keys_fsm.erl:104: Expression produces a value of type {_,'ok'}, but this value is unmatched
riak_kv_pb_bucket.erl:129: Expression produces a value of type {_,'ok'}, but this value is unmatched
riak_kv_pb_bucket.erl:136: Expression produces a value of type {_,'ok'}, but this value is unmatched
riak_kv_pipe_listkeys.erl:102: Expression produces a value of type {_,'ok'}, but this value is unmatched