Remove obsolete(?) comments

`riak_pb_code.erl` and `riak_pb_kv_codec.erl` both refer to `riakc_pb` which does not appear to exist anywhere, perhaps a precursor to `riakc_pb_socket` but the function names do not align.

```
-export([encode/1,      %% riakc_pb:encode
         decode/2,      %% riakc_pb:decode
         msg_type/1,    %% riakc_pb:msg_type
         msg_code/1,    %% riakc_pb:msg_code
```
