overviewControls: Remove transitions before updating adjustment value

The workspace switcher blocks state updates while the indicator is animating. Since commit 9c1940ef the indicator is considered to be animating when the workspace adjustment's value doesn't equal the active workspace.

There is one case though where this breaks badly: When a workspace is inserted before the active one, the adjustment's upper and value properties are changed without transitions. But if that change happens while there's an ongoing transition to the previously active workspace, the value gets out of sync with the active workspace and we end up blocking state updates indefinitely.

Fix this by removing any transitions before setting the adjustment value.

Fixes #2625 (closed)