Remove text of 'back-button' in header

src/app/components/shared/header/header.component.html

@@ -2,7 +2,6 @@
   <mat-toolbar-row>
     <button *ngIf="router.url !== '/home'" (click)="goBack()" mat-icon-button>
       <mat-icon aria-label="Go back">keyboard_arrow_left</mat-icon>
-      <span> {{ 'header.back' | translate }}</span>
     </button>
     <span class="fill-remaining-space"></span>
     <span *ngIf="router.url !== '/home'" class="app-title" (click)="goToHomepage()">ARSnova</span>