variant: remove useless assertion

* data/variant.hh (move): Remove precondition assertion which is
ensured by the first call of the body (this precondition is also one
of "build").

data/variant.hh

@@ -178,8 +178,7 @@ m4_define([b4_variant_define],
     template <typename T>
     void
     move (self_type& other)
-    {]b4_parse_assert_if([
-      YYASSERT (!yytname_);])[
+    {
       build<T> ();
       swap<T> (other);
       other.destroy<T> ();