Stay in game if no side could be found

Having no side is the definition of an observer. No need to add new
client-server messages for this. This is not a new feature either.

src/gui/dialogs/multiplayer/mp_join_game.cpp

@@ -125,8 +125,6 @@ bool mp_join_game::fetch_game_config()
 			}
 
 			has_scenario_and_controllers = true;
-		} else if(revc.has_child("became_observer")) {
-			observe_game_ = true;
 		}
 	}
 
@@ -185,7 +183,8 @@ bool mp_join_game::fetch_game_config()
 	}
 
 	if(!side_choice) {
-		return false;
+		observe_game_ = true;
+		return true;
 	}
 
 	// If the client is allowed to choose their team, do that here instead of having it set by the server

src/server/game.cpp

@@ -1409,14 +1409,6 @@ bool game::add_player(const socket_ptr& player, bool observer)
 	player_connections_.find(user)->info().set_status((observer) ? player::OBSERVING : player::PLAYING);
 	DBG_GAME << debug_player_info();
 
-	// If a player was switched to an observer, notify them of the fact.
-	if(became_observer) {
-		simple_wml::document became_observer_notif;
-		became_observer_notif.root().add_child("became_observer");
-
-		send_to_player(player, became_observer_notif);
-	}
-
 	// Send the user the game data.
 	send_to_player(player, level_);