Remove error for @typescript-eslint/no-namespace (#7803)

Declare namespaces are supported by babel now, and babel will throw with a nice error message for non-declare namespaces, so this rule is unnecessary. Closes #7651.

packages/eslint-config-react-app/index.js

@@ -81,7 +81,6 @@ module.exports = {
         '@typescript-eslint/consistent-type-assertions': 'warn',
         'no-array-constructor': 'off',
         '@typescript-eslint/no-array-constructor': 'warn',
-        '@typescript-eslint/no-namespace': 'error',
         'no-use-before-define': 'off',
         '@typescript-eslint/no-use-before-define': [
           'warn',

test/fixtures/typescript/src/App.ts

@@ -27,4 +27,11 @@ function propertyDecorator(target: any, key: string) {
   };
 }
 
+declare namespace MyNamespace {
+  interface MyType {
+    foo: string;
+    bar: (n: number) => void;
+  }
+}
+
 export default App;