ember: Fix no-unnecessary-class error (#26392)

types/ember/index.d.ts

@@ -2368,7 +2368,7 @@ declare module 'ember' {
             function Exception(message: string): void;
             class SafeString {
                 constructor(str: string);
-                static toString(): string;
+                toString(): string;
             }
             function parse(string: string): any;
             function print(ast: any): void;

types/ember/tslint.json

@@ -24,8 +24,6 @@
         "only-arrow-functions": false,
         "no-submodule-imports": false,
 
-        "no-unnecessary-class": false,
-
         // false positives
         "unified-signatures": false
     }