[wayland] fix crash on startup with lv3:ralt_alt XKB option

With lv3:ralt_alt ("Right Alt never chooses 3rd level") option set, we
get more layouts from libxkbcommon than it was configured, see:
https://github.com/xkbcommon/libxkbcommon/issues/262
It might be correct lib's behavior, still.

The extra layouts are redundant, so we strip them out from usual usage.

BUG: 440027