parser: remove XkbConfig rule

parser: remove XkbConfig rule

This rule allows you to write file maps as:
    xkb_keycodes
    <BLA> = 5;
    [...]
instead of the usual format which is:
    xkb_keycodes {
        <BLA> = 5;
        [...]
    };

This is not documented, It is also not used in xkeyboard-config, and I
have never run into it otherwise. It also only allows one map per file.

It *might* be used in some obscure place, but probably nothing we should
care about; the simplified grammar is more useful for us now.

Signed-off-by: Ran Benita <ran234@gmail.com>