fix(nav): drop active-board icon fill override

Rely on nextcloud-vue tertiary button default (--color-main-text)
instead of overriding the active board actions icon fill.

Fixes #7539

Author: xhon-pelushi

src/components/navigation/AppNavigationBoard.vue

@@ -534,10 +534,4 @@ export default {
 	.forced-active {
 		box-shadow: inset 4px 0 var(--color-primary-element);
 	}
-
-	:deep(.active) {
-		.material-design-icon svg {
-			fill: var(--color-primary-element-text);
-		}
-	}
 </style>