pc: completions - do not add [] for ... derives TC@@ (#23811)

Currently it incorectly adds completion members with square brackets.
Exmaple:
```scala
class X derives CanEqua@@ // returns `CanEqual[@@]` and `CanEqual`
                          // should return only `CanEqual`
```
[Cherry-picked 17f1803][modified]

Author: WojciechMazur