docs: use icon button in input clear example (#22732)

Remove the extraneous `mat-button` directive.

Author: bampakoa

src/components-examples/material/input/input-clearable/input-clearable-example.html

@@ -1,7 +1,7 @@
 <mat-form-field class="example-form-field">
   <mat-label>Clearable input</mat-label>
   <input matInput type="text" [(ngModel)]="value">
-  <button mat-button *ngIf="value" matSuffix mat-icon-button aria-label="Clear" (click)="value=''">
+  <button *ngIf="value" matSuffix mat-icon-button aria-label="Clear" (click)="value=''">
     <mat-icon>close</mat-icon>
   </button>
 </mat-form-field>