Cut invalid overly-specific type cast

Cut invalid overly-specific type cast

The Vala compiler now correctly warns that typeof(Foo<Bar>) is invalid,
so this stops pretending we can be that specific.

(The generated C code can't make a GValue as specific as the above Vala
code fragment suggests; historically, the compiler would let you get
away with this, likely with the false assumption that the generic type
would ever be considered again.)