Drop the Protobuf dependency of gemrpc

Description

Even if rcpsvc (now gemrpc) has been integrated into cmsgemos and the main serialization library is cereal, the Protobuf code was never removed. It is however useless in the software stack, complexifies the compilation and installation, and maybe slows down the RPC calls.

Possible fixes

Remove any leftover of Protobuf still used to encapsulate the cereal binary archive. A small cereal-serialized structure will be required to signal whether an exception was thrown or not.