[api] Remove ProtoService base class

ProtoService was an abstract interface with 6 pure virtual methods,
but APIConnection was the only concrete implementation. Move all
functionality directly into APIConnection and remove the unnecessary
virtual dispatch and vtable overhead.

script/api_protobuf/api_protobuf.py

@@ -2958,7 +2958,7 @@ static const char *const TAG = "api.service";
 
     class_name = "APIServerConnectionBase"
 
-    hpp += f"class {class_name} : public ProtoService {{\n"
+    hpp += f"class {class_name} {{\n"
     hpp += " public:\n"
 
     # Add logging helper method declarations