docs(forms): fix duplicate validator reference titles in AbstractControl

The removeValidators and hasValidator methods both had identical "Reference to a ValidatorFn" section titles, causing duplicate entries in the API documentation table of contents.

packages/forms/src/model/abstract_model.ts

@@ -865,8 +865,6 @@ export abstract class AbstractControl<
    *
    * @usageNotes
    *
-   * ### Reference to a ValidatorFn
-   *
    * ```ts
    * // Reference to the RequiredValidator
    * const ctrl = new FormControl<string | null>('', Validators.required);
@@ -911,8 +909,6 @@ export abstract class AbstractControl<
    *
    * @usageNotes
    *
-   * ### Reference to a ValidatorFn
-   *
    * ```ts
    * // Reference to the RequiredValidator
    * const ctrl = new FormControl<number | null>(0, Validators.required);