Remove extant reference to the `symbols/extras' directory

[xorg/xkeyboard-config] / symbols / terminate

partial default modifier_keys
xkb_symbols "ctrl_alt_bksp" {
    key <BKSP> {
        type="CTRL+ALT",
        symbols[Group1] = [ NoSymbol, NoSymbol, NoSymbol, NoSymbol, Terminate_Server ]
    };
};