refactor: select component: remove unused disabled flag

refs #483

src/app/components/generic-select/generic-select.component.html

 <mat-form-field>
-    <mat-select [id]="selectId" [placeholder]="label" [(value)]="selectedValue" [multiple]="isMultiple"
-        [disabled]="isDisabled">
+    <mat-select [id]="selectId" [placeholder]="label" [(value)]="selectedValue" [multiple]="isMultiple">
         <mat-select-trigger *ngIf="isMultiple">
             {{ selectedValue && selectedValue[0] ? entryLabel(selectedValue[0]) : '' }}
             <span *ngIf="selectedValue?.length > 1" class="multiple-selection-label">

src/app/components/select-field-line/select-field-line.component.ts

@@ -34,10 +34,6 @@ export class SelectFieldLineComponent implements OnInit {
         return this._select.multiple;
     }
 
-    public get isDisabled(): boolean {
-        return this._select.disabled;
-    }
-
     public get entries(): SelectEntry[] {
         if (! this._select) {
             return [];

src/app/formulaire/elements/select/select-field.ts

@@ -21,9 +21,6 @@ export abstract class SelectField extends Field {
     /** if true, user can select multiple values */
     protected _multiple = false;
 
-    /** if true, select box is grayed out */
-    public disabled = false;
-
     /** string to build the select entries IDs from */
     protected _entriesBaseId: string;