Revert "Add callIfNull support"

This reverts commit ddeeb380a7.

index.js

@@ -5,13 +5,9 @@ const combinator = require("@validatem/combinator");
 // TODO: Document that this *exposes* context
 
 module.exports = function dynamic(ruleGenerationFunction) {
-	let validator = combinator((value, applyValidators, context) => {
+	return combinator((value, applyValidators, context) => {
 		let rules = ruleGenerationFunction(value, context);
 
 		return applyValidators(value, rules);
 	});
-
-	validator.callIfNull = true;
-
-	return validator;
 };