refactor(forms): remove ngForm element selector (#33058)

Removes the deprecated `ngForm` element selector and all of the code related to it.

BREAKING CHANGES:
* `<ngForm></ngForm>` can no longer be used as a selector. Use `<ng-form></ng-form>` instead.
* The `NgFromSelectorWarning` directive has been removed.
* `FormsModule.withConfig` has been removed. Use the `FormsModule` directly.

PR Close #33058

tools/public_api_guard/forms/forms.d.ts

@@ -327,8 +327,6 @@ export declare class FormGroupName extends AbstractFormGroupDirective implements
 }
 
 export declare class FormsModule {
-    static withConfig(opts: { warnOnDeprecatedNgFormSelector?: 'never' | 'once' | 'always';
-    }): ModuleWithProviders<FormsModule>;
 }
 
 export declare class MaxLengthValidator implements Validator, OnChanges {
@@ -397,11 +395,6 @@ export declare class NgForm extends ControlContainer implements Form, AfterViewI
     updateModel(dir: NgControl, value: any): void;
 }
 
-/** @deprecated */
-export declare class NgFormSelectorWarning {
-    constructor(ngFormWarning: string | null);
-}
-
 export declare class NgModel extends NgControl implements OnChanges, OnDestroy {
     readonly asyncValidator: AsyncValidatorFn | null;
     readonly control: FormControl;