Fix directives preventing scope to "refine" themselves properly

The `Scope.refine` method, which is in charge of ensuring the scope are
"minimal" (don't contain unecessary conditions), has a bug whereby if we
refine with a directive, we don't minimalize the resulting code
properly.

This can lead a query plan to include conditions of the form:
```
... on I {
  ... on E @foo {
    v
  }
}
```
where `E` is an implementation of interface `I` (and as such the
condition on `I` is redundant). This is harmless in many situations but
this can happen in a query to a subgraph that does not know of interface
`I`, thus leading to an invalid query.

The fix is simply to remove the special casing for directives that
leads to not properly minimizing the scope. That condition was an
unfortunate left-over of a previous version of the code that wasn't
properly updated.

Fixes: #801

gateway-js/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 -  _Nothing yet! Stay tuned!_
 
+## v0.28.1
+
+- Fix plan querying a subgraph with an interface it doesn't know due to directives ([#801](https://github.com/apollographql/federation/issues/801)).
+
 ## v0.28.0
 
 - Expand the range of supported `node` versions in the package's `engines` specifier to include the now-tested Node.js `16`. [PR #713](https://github.com/apollographql/federation/pull/713)