Remove explicit dependency on riak_pb
Remove explicit dependency on riak_pb

riak_search depends on riak_api, which has a rebar dependency on
riak_pb, so this one is not needed. Moreover, it's conflicting with the
one in riak_api, causing compilation problems in riak.

basho/riak_search,basho/riak_search,wwezhuimeng/riak_search,xuejianhai/riak_search,xuejianhai/riak_search,wwezhuimeng/riak_search