Simplify "Playtime" column header in Staff Playtime view.

frontend/src/app/pages/head-mod/staff-pt/staff-pt.component.html

@@ -25,7 +25,7 @@
         </ng-container>
 
         <ng-container matColumnDef="playtime">
-          <th mat-header-cell *matHeaderCellDef> Playtime (h:mm)</th>
+          <th mat-header-cell *matHeaderCellDef> Playtime</th>
           <td mat-cell *matCellDef="let row"> {{ minutesToHm(row.playtime) }}</td>
         </ng-container>