Remove the icons from the application menu. Why remove?

- They are inconsistently applied, some actions had icons, some newer ones don't. Good luck coming up with a sensible icon for everything
- Menu icons don't seem to have a place in modern UI: for example, GNOME, MacOS have stopped showing these a long time ago (see https://github.com/bitcoin/bitcoin/pull/16584#issuecomment-521195090)
- Less bikeshedding opportunity about "what should the icon for this be"

Removed icons:

```
/icons/quit            res/icons/quit.png
/icons/about           res/icons/about.png
/icons/about_qt        res/icons/about_qt.png
/icons/options         res/icons/configure.png
/icons/key             res/icons/key.png
/icons/verify          res/icons/verify.png (also .svg)
/icons/debugwindow     res/icons/debugwindow.png
/icons/open            res/icons/open.png
/icons/info            res/icons/info.png
/icons/filesave        res/icons/filesave.png
```
I checked that these icons are used nowhere else.

Removed from the menu not removed from the repository, because still referenced by other parts of the code:
```
/icons/lock_closed
/icons/edit
/icons/address-book
/icons/send
```

ninja all