Remove explicit CSS import for vis-timeline

The styles are already added to the DOM (which is not a good idea in terms of CSP support). With the newest vis release 8.x, the CSS files are no longer exported and thus cannot be added any longer. Nevertheless, the existing solution works fine for now.

Author: MrSerth

app/javascript/vis.scss

@@ -1,7 +1,3 @@
-// CSS
-
-@import 'vis-timeline/dist/vis-timeline-graph2d';
-
 html[data-bs-theme="dark"] {
   // Color overwrites for dark mode