Remove hints and help on shutdown

Remove hints and help on shutdown

Previously, if you disabled VimFx by using the `f` command to click VimFx's
"Disable" button in the Add-ons Manager, the hint markers would be left behind
(though losing their styling). This meant that the entire page would be covered
by a transparent element impossible to remove (other than closing the tab) and
impossible to click through.

A similar thing happened if you left the help dialog open in another tab while
disabling VimFx.

This commit makes sure that both those pieces of UI are removed on shutdown.