Use default empty.Empty message instead of EmptyMsg

[mancabizjak]

This commit removes EmptyMsg from proto/messages.proto. Instead, we use a pre-defined default type Empty to avoid unnecessary redefinition of empty message type in the API.

From the above package docs about the Empty type:

A generic empty message that you can re-use to avoid defining duplicated empty messages in your APIs. A typical example is to use it as the request or the response type of an API method.

In addition, empty message is no longer a part of the Message message type, since none of the currently implemented schemes uses it anymore.