Please do not use `popup_clear()`

[Bakudankun]

This plugin uses popup_clear() in hover.vim, but it is "Emergency solution to a misbehaving plugin" (:h popup_clear()) and it forcibly closes every popups including popups that other plugins make.

Please somehow remember the popupwin's ID and close with popup_close(). I think there is no problem in test scripts.

[vimpostor]

Seems this is a regression introduced in #208