Masked hotkey fix

This is based on top of https://github.com/CleverRaven/Cataclysm-DDA/pull/7291, which isn't merged yet.

If a user adds a local hotkeys that masks the default hotkey, removing the hotkey results in unexpected behavior. That hotkey enters a state where only local hotkey keybindings may be added -- the user is unable to return the hotkey back to its original state (it shows up highlighted in green).

This patch treats user created local hotkeys specially. If one keybindings on such a hotkey are removed, that hotkey is deleted, reverting back to the hotkey in the default context.