Revert "Drop redundant butNot = Param clause in isAnchor"

This reverts commit 9d88c80.
Closes scala#21521

The `ClassTypeParamCreationFlags` include both `TypeParam` and `Deferred`.
In effect, a class type parameter was incorrectly considered as an anchor.

For a failing example, one can try asserting:
```scala
|| sym.is(Deferred).ensuring(_ == sym.is(Deferred, butNot = Param))
```
in `ImplicitRunInfo#isAnchor` and a test with `summon[Ordering[Int]]`.

In that example, at least,
the flags happen to be set by `Scala2Unpickler#readDisambiguatedSymbol`
src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala:562.

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -636,7 +636,7 @@ trait ImplicitRunInfo:
   private def isAnchor(sym: Symbol) =
     sym.isClass && !isExcluded(sym)
     || sym.isOpaqueAlias
-    || sym.is(Deferred)
+    || sym.is(Deferred, butNot = Param)
     || sym.info.isMatchAlias
 
   private def computeIScope(rootTp: Type): OfTypeImplicits =