GH-1266 Improve and clean up SimpleFunctionRegistry

Resolves #1266

spring-cloud-function-core/src/main/java/org/springframework/cloud/function/core/FunctionInvocationHelper.java

@@ -31,14 +31,6 @@ public interface FunctionInvocationHelper<I> {
 		return true;
 	}
 
-	/**
-	 * @deprecated Use {@link #isRetainOutputAsMessage(I)}
-	 */
-	@Deprecated
-	default boolean isRetainOuputAsMessage(I input) {
-		return isRetainOutputAsMessage(input);
-	};
-
 	default I preProcessInput(I input, Object inputConverter) {
 		return input;
 	}