Make it possible to clear / remove Game Controller Mappings for a device

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe.

I am using vJoy to expose a virtual joystick, with several axes and a fair number of buttons - but very much a joystick, _not_ a gamepad.

However, the game controller database that's included with AntiMicroX includes an entry for "vJoy Device" which matches all vJoy devices. This is arguably an error in the database, and I've raised [an issue there](https://github.com/mdqinc/SDL_GameControllerDB/issues/859).

So when I opened AntiMicroX, I did not see the full set of axes and buttons - I saw only what the game controller mapping included, per the matching erroneous database entry.

I figured I might be able to fix that on the "Controller Mapping" screen. But there, I can see the Mappings, and change them by selecting an entry to change and then moving an axis or button ... but I can't seem to delete a mapping that I don't want to use.

There is the "Discard" button which sounds promising: clicking it opens a dialog warning me, "If discarded, the controller will be reverted to a joystick once you refresh all joysticks."

But if I proceed to discard and then refresh the joysticks, I just get the same erroneous gamepad mapping, rather than the more raw underlying joystick that exposes what I actually need.

Fortunately, I was able to work around the issue by removing the "vJoy Device" entry from `C:\Program Files\AntiMicroX\share\antimicrox\gamecontrollerdb.txt`,which now allows me to see the vJoy device as the raw joystick that I need.

However, that seems like a rather unsatisfying workaround, and it seems like being able to remove the mappings on the "Controller Mapping" page might be a good idea not just for my case, but also for any case where a game controller has been misdescribed - or indeed if someone simply wants to access the underlying raw axes and buttons of the device.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_