Removed redundant `retypedMethod` in `Duplicators`

It was never used since its introduction in 3ee6b3653 by @dragos.

Review by @dragos or @axel22 or @paulp.
SI-7343 Fixed phase ordering in specialization

Specialization rewires class parents during info transformation, and
the new info then guides the tree changes. But if a symbol is created
during duplication, which runs after specialization, its info is not
visited and thus the corresponding tree is not specialized.

One manifestation is the following:
```
object Test {

  class Parent[@specialized(Int) T]

  def spec_method[@specialized(Int) T](t: T, expectedXSuper: String) = {
     class X extends Parent[T]()
     // even in the specialized variant, the local X class
     // doesn't extend Parent$mcI$sp, since its symbol has
     // been created after specialization and was not seen
     // by specialzation's info transformer.
     ...
  }
}
```
We can fix this by forcing duplication to take place before specialization.

Review by @dragos, @paulp or @axel22.
Merge pull request #2651 from VladUreche/issue/7343-2

SI-7343 Fixed phase ordering in specialization