plugins/overview: remove middle click to close virtual desktop
This is a hidden destructive action[1] in a context where users are not primed to expect it. It's also largely unnecessary since the delegates already show a delete button on hover, and deleting virtual desktops isn't such a common task that it benefits from an accelerator. Let's remove it for safety.
[1] It's destructive because user-created state on the deleted virtual desktop is lost--with no confirmation or undo, to boot.
BUG: 484999 FIXED-IN: 6.1