refactor(forms): remove unused internal function (#46479)

The `isFormControlOptions` function is never used, and can be safely removed as it is not a public API.

PR Close #46479

packages/forms/src/form_builder.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {inject, Injectable, InjectionToken} from '@angular/core';
+import {inject, Injectable} from '@angular/core';
 
 import {AsyncValidatorFn, ValidatorFn} from './directives/validators';
 import {ReactiveFormsModule} from './form_providers';
@@ -23,14 +23,6 @@ function isAbstractControlOptions(options: AbstractControlOptions|{[key: string]
        (options as AbstractControlOptions).updateOn !== undefined);
 }
 
-function isFormControlOptions(options: FormControlOptions|{[key: string]: any}|null|
-                              undefined): options is FormControlOptions {
-  return !!options &&
-      (isAbstractControlOptions(options) ||
-       (options as FormControlOptions).initialValueIsDefault !== undefined ||
-       (options as FormControlOptions).nonNullable !== undefined);
-}
-
 /**
  * ControlConfig<T> is a tuple containing a value of type T, plus optional validators and async
  * validators.