[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

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/IrTypeSystemContext.kt

@@ -94,12 +94,6 @@ interface IrTypeSystemContext : TypeSystemContext, TypeSystemCommonSuperTypesCon
             else -> error("Type $this has no arguments")
         }
 
-    override fun KotlinTypeMarker.getArguments(): List<TypeArgumentMarker> =
-        when (this) {
-            is IrSimpleType -> arguments
-            else -> error("Type $this has no arguments")
-        }
-
     override fun KotlinTypeMarker.asTypeArgument() = this as IrTypeArgument
 
     override fun CapturedTypeMarker.lowerType(): KotlinTypeMarker? = error("Captured Type is not valid for IrTypes")