The ALT key in Unity opens the Unity HUD (just try it outside of the game). It offers a search for every entry in the program menu. I'm not sure how to uninstall it, though. The HUD itself is contained in the package
hud, but removing it will likely give the alt key another function. But I'll show you how to change the shortcut.
Since I don't know what the English localization for the program needed for keyboard changes is called, I give you the terminal way (also graphical, just to start the application!)
Type in terminal:
- Click on the Shortcut tab.
- Find the section Starter and click on it.
- Change or delete the shortcut for calling the HUD (Standard: ALT L)
This should work for you. Remember the shortcut is (more or less?) global. So setting it to CTRL+S or CTRL+C or something like that would not be good.