[Kickoff] Remove arrow indicator
We generally don't use arrows in sidebars unless it's a subcategory that opens another list (like settings/discover sidebar). Other launchers like kickerdash also lack arrows. We also get more space for labels before the sidebar has to expand.
The main concern is that the users might have harder time differentiating apps and categories but I don't think it's an issue, since apps have labels and hovering over them blanks the right pane.
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(PS. I also wanted to hide apps from the left pane, but that's probably too controversial to be in this MR)