Ignore result of send_output.

Resolves:

riak_kv_mrc_pipe.erl:1070: Expression produces a value of type ['ok' | {'error','sink_died' | 'timeout' | [any()]}], but this value is unmatched
Ignore the result from ack_keys.

Resolves:

riak_kv_pipe_index.erl:88: Expression produces a value of type {_,'ok'}, but this value is unmatched