Remove backend tests on old inference

Also remove any mentions of NewInference, and rename some tests.

compiler/testData/codegen/box/regressions/kt33638.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // WITH_RUNTIME
 
 class Holder(val list: List<String>?)

compiler/testData/codegen/box/regressions/kt35914.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // WITH_RUNTIME
 
 class Inv<T>

compiler/testData/codegen/box/regressions/noAssertionsWhenNullableTypeParameterReplacedWithIntersectionType.kt

@@ -1,5 +1,3 @@
-// !LANGUAGE: +NewInference
-
 class Recursive<T : Recursive<T>> : Generic<PlaceHolder<T>>, MainSupertype
 open class Simple<T> : Generic<T>, MainSupertype
 
@@ -23,4 +21,4 @@ fun <T> select(x: T?, y: T): T = y
 fun box(): String {
     takeElement(null, Simple())
     return "OK"
-}
+}