vmod: remove support for resolving integer to virtual modifier

vmod: remove support for resolving integer to virtual modifier

This is only relevant to the virtualModifier= statement in interpret
statements in xkb_compat. The current code allows to write e.g.
    virtualModifier = 4
to get the virtual modifier whose index happens to be 4 (possibly
declared in other files or sections, i.e. xkb_types). Doing this is
undeterministic, will break without notice, and not used anywhere.
Don't allow it.

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