Remove protobuffs dependency from riak_core.

The riak_core.proto was only used in riak_kv's handoff, and has no
references elsewhere in riak_core. It is appropriately moving to
riak_kv.

/cc @Vagabond @evanmcc @jrwest
Merge pull request #382 from basho/refactor/sdc-rm-protobuffs

Remove protobuffs dependency from riak_core.