Do not give value to "disabled" attribute

The presence of the "disabled" attribute
in a HTML tag is enough to disable it.
It does not take a value.

src/main/webapp/app/exercise/exercise-details/exercise-details.component.html

@@ -93,8 +93,9 @@
                                 </button> -->
 
                                 <button *ngIf="exercise.type == exerciseType.COLLECTION" type="button" class="btn btn-outline-secondary"
-                                    style="display: block; margin-bottom: 5px;" disabled="true"
-                                    jhiTranslate="exercise.details.allExercises">
+                                    style="display: block; margin-bottom: 5px;"
+                                    jhiTranslate="exercise.details.allExercises"
+                                    disabled>
                                 </button>
 
                                 <button type="button" class="btn btn-outline-secondary" style="display: block;"
@@ -327,10 +328,11 @@
                                 <p style="text-align: left; margin-top: 5px;"><strong jhiTranslate="exercise.export.export"></strong>
                                 </p>
                                 <button *ngIf="exercise.type == exerciseType.COLLECTION" type="button" class="btn btn-outline-secondary"
-                                    style="float: left; margin-right: 5px; margin-top: 5px;" disabled="true"
-                                    jhiTranslate="exercise.export.latex">
+                                    style="float: left; margin-right: 5px; margin-top: 5px;"
+                                    jhiTranslate="exercise.export.latex"
+                                    disabled>
                                 </button>
-                                
+
                                 <a href="#" class="btn btn-outline-secondary" role="button" aria-pressed="true"
                                     style="float: left; margin-right: 5px; margin-top: 5px;"
                                     jhiTranslate="exercise.export.download"></a>