[REG2.063] Issue 10628 - spurious "hidden by" deprecation warning

http://d.puremagic.com/issues/show_bug.cgi?id=10628

In old days, the code was added to fix [regression 3740](http://d.puremagic.com/issues/show_bug.cgi?id=3740).
But from 2.063, enhancement 7511 (#1676) has introduced more forward reference and its resolving.

Today, forward reference is correctly resolved with `Dsymbol::scope`, so "re-run semantic() in order to set the class's vtbl[]" is not necessary.