Remove the unneeded hardcoding of the Alt+F4 hotkey which causes 0AD on linux to exit unintentionally after receiving the window focus from another window which was closed with that hotkey (with multiple tiling window managers). Reviewed by Philip, tested by Imarok and Itms, fixes #4272.
The hotkey introduced in r7933 was likely a workaround for the custom windows SDL (wsdl) implementation which is no longer in use.