Minor: cleanup + support variance in ErrorListDiagnosticListRenderer

1. Support variance in ErrorListDiagnosticListRenderer.
   I will make some diagnostics to have covariant type arguments in the
   next commits
2. Drop unused code

Review: https://jetbrains.team/p/kt/reviews/12750/timeline

core/compiler.common/src/org/jetbrains/kotlin/resolve/multiplatform/ExpectActualCompatibility.kt

@@ -105,11 +105,3 @@ val ExpectActualCompatibility<*>.isCompatibleOrWeaklyIncompatible: Boolean
 
 val ExpectActualCompatibility<*>.compatible: Boolean
     get() = this == ExpectActualCompatibility.Compatible
-
-@OptIn(ExperimentalContracts::class)
-fun ExpectActualCompatibility<*>.isIncompatible(): Boolean {
-    contract {
-        returns(true) implies (this@isIncompatible is ExpectActualCompatibility.Incompatible<*>)
-    }
-    return !compatible
-}