Fix isAlwaysPure

The condition on capturing types did not make sense. In a type T^{} with an empty capture set
`T` can still be a type variable that's instantiated to a type with a capture set. Instead,
T^cs is always pure if T is always pure. For instance `List[T]^{p}` is always pure. That's
important in the context of the standard library, where such a type usually results from
an instantiation of a type variable such as `C[T]^{p}`.

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -282,8 +282,6 @@ extension (tp: Type)
       val sym = tp.typeSymbol
       if sym.isClass then sym.isPureClass
       else tp.superType.isAlwaysPure
-    case CapturingType(parent, refs) =>
-      parent.isAlwaysPure || refs.isAlwaysEmpty
     case tp: TypeProxy =>
       tp.superType.isAlwaysPure
     case tp: AndType =>