input_common/input_mapping: Remove const from return value

Top-level const on a return by value can inhibit move semantics, and is
unnecessary.

src/input_common/input_mapping.cpp

@@ -19,7 +19,7 @@ void MappingFactory::BeginMapping(Polling::InputType type) {
     second_axis = -1;
 }
 
-[[nodiscard]] const Common::ParamPackage MappingFactory::GetNextInput() {
+Common::ParamPackage MappingFactory::GetNextInput() {
     Common::ParamPackage input;
     input_queue.Pop(input);
     return input;