Simplify code

Author: ondrejmirtes

src/Rules/FunctionDefinitionCheck.php

@@ -712,18 +712,12 @@ private function checkImplicitlyNullableType(
 		if ($type instanceof Identifier && strtolower($type->name) === 'mixed') {
 			return null;
 		}
-		if ($type instanceof Name && $type->toLowerString() === 'mixed') {
-			return null;
-		}
 
 		if ($type instanceof UnionType) {
 			foreach ($type->types as $innerType) {
 				if ($innerType instanceof Identifier && strtolower($innerType->name) === 'null') {
 					return null;
 				}
-				if ($innerType instanceof Name && $innerType->toLowerString() === 'null') {
-					return null;
-				}
 			}
 		}