gtk: rely on default marshallers
Similar to previous removals of g_cclosure_marshal_VOID__VOID we can remove other marshallers for which are a simple G_TYPE_NONE with single parameter. In those cases, GLib will setup both a c_marshaller and va_marshaller for us. Before this commit, we would not get a va_marshaller because the c_marshaller is set.
Related to Initiatives#10