Do not force use of us layout if (modifiers & IBUS_MOD5_MASK) != 0

even if klass->use_us_layout is false, us layout was forced when
“(modifiers & IBUS_MOD5_MASK) != 0”. Therefore, all characters which
can be typed by the right Alt key + some other key did not work in
ibus-m17n engines, even if the option to use us layout was set to
false.

For example, with the German keyboard layout, one can type a € (Euro sign)
by typing Alt_R+e.

I think there is no need to get the underlying character from the US
layout for these characters, it is better to just use them unchanged.