[FIR/NI] Refactor type variable gathering from lambda types

Motivation:
- drop getArguments from type context as a duplicate of getArgumentList
- reduce the number of collection allocations in getAllDeeplyRelatedTypeVariables

Additional minor improvements, test data fixes

core/descriptors/src/org/jetbrains/kotlin/types/checker/ClassicTypeSystemContext.kt

@@ -159,11 +159,6 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
         return this.arguments[index]
     }
 
-    override fun KotlinTypeMarker.getArguments(): List<TypeArgumentMarker> {
-        require(this is KotlinType, this::errorMessage)
-        return this.arguments
-    }
-
     override fun TypeArgumentMarker.isStarProjection(): Boolean {
         require(this is TypeProjection, this::errorMessage)
         return this.isStarProjection