FE: remove ? enhancement code that is never used

Using `starProjectionType` to enhance `?` is pointless because
 1. we explicitly ignore nullability annotations for `?` below;
 2. mutability is irrelevant;
 3. `starProjectionType` never returns references to other type
    parameters due to how the type substitutor works.

core/descriptors.jvm/src/org/jetbrains/kotlin/load/java/typeEnhancement/signatureEnhancement.kt

@@ -251,9 +251,6 @@ private class SignatureParts(
     override fun KotlinTypeMarker.isEqual(other: KotlinTypeMarker): Boolean =
         containerContext.components.kotlinTypeChecker.equalTypes(this as KotlinType, other as KotlinType)
 
-    override val TypeParameterMarker.starProjectedType: KotlinType
-        get() = (this as TypeParameterDescriptor).starProjectionType()
-
     override val TypeParameterMarker.isFromJava: Boolean
         get() = this is LazyJavaTypeParameterDescriptor
 }