Remove explicit std::move in prepare_mission_menu_state return

clang warns that this interferes with copy elision.  The generated code
is slightly worse after following clang's advice and removing this, but
this is not a hot path, so take the hit to silence clang instead of
complicating the source.

Fixes: d97afc2ad5 ("Retain directory structure in New Game dialog")

similar/main/mission.cpp

@@ -1259,7 +1259,7 @@ static mission_menu_create_state_ptr prepare_mission_menu_state(const mission_li
 		create_state.initial_selection = e.idx;
 		mission_name_to_select = nullptr;
 	}
-	return std::move(p);
+	return p;
 }
 
 int select_mission(int anarchy_mode, const char *message, window_event_result (*when_selected)(void))