[light classes] drop old light classes and backend: iteration #2

drop javaFileStub and fix tests

^KT-48773

compiler/testData/asJava/lightClasses/nullabilityAnnotations/OverrideAnyWithUnit.kt

@@ -8,5 +8,4 @@ class C : Base {
     override fun foo(): Unit {}
 }
 
-// LAZINESS:NoLaziness
 // FIR_COMPARISON

compiler/testData/asJava/lightClasses/nullabilityAnnotations/UnitAsGenericArgument.kt

@@ -7,5 +7,4 @@ interface Base<T> {
 class C : Base<Unit> {
     override fun foo(t: Unit) {}
 }
-// LAZINESS:NoLaziness
 // FIR_COMPARISON