change flatten to Array#flat (Node 10.x EOL)

index.js

@@ -3,7 +3,6 @@
 const asExpression = require("as-expression");
 const defaultValue = require("default-value");
 const assureArray = require("assure-array");
-const flatten = require("flatten");
 const arrayUnion = require("array-union");
 
 const ValidationError = require("@validatem/error");
@@ -17,7 +16,7 @@ function containsRules(rules) {
 		return false;
 	} else {
 		// TODO: Switch to `Array#flat` once Node 10.x goes EOL (April 2021)
-		let flattenedRules = flatten(assureArray(rules));
+		let flattenedRules = assureArray(rules).flat();
 
 		if (!flattenedRules.some((rule) => rule != null)) {
 			return false;

package.json

@@ -19,8 +19,7 @@
     "array-union": "^2.1.0",
     "as-expression": "^1.0.0",
     "assure-array": "^1.0.0",
-    "default-value": "^1.0.0",
-    "flatten": "^1.0.3"
+    "default-value": "^1.0.0"
   },
   "devDependencies": {
     "@validatem/core": "^0.3.1",