Add download button on pool ranking and mempool graphs

frontend/src/app/components/pool-ranking/pool-ranking.component.html

@@ -25,7 +25,7 @@
 
   <div class="card-header" *ngIf="!widget">
     <span i18n="mining.mining-pool-share">Mining pools share</span>
-    <button #saveChart class="btn" style="position: absolute; right: 30px" (click)="onSaveChart()">
+    <button class="btn" style="position: absolute; right: 30px" (click)="onSaveChart()">
       <fa-icon [icon]="['fas', 'download']" [fixedWidth]="true"></fa-icon>
     </button>