Remove backend tests on old inference

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

compiler/testData/codegen/box/mixedNamedPosition/defaults.kt

@@ -1,5 +1,3 @@
-// !LANGUAGE: +NewInference +MixedNamedArgumentsInTheirOwnPosition
-
 fun foo(
     p1: Int = 1,
     p2: String = "2",

compiler/testData/codegen/box/mixedNamedPosition/simple.kt

@@ -1,4 +1,4 @@
-// !LANGUAGE: +NewInference +MixedNamedArgumentsInTheirOwnPosition
+// !LANGUAGE: +MixedNamedArgumentsInTheirOwnPosition
 
 fun foo(
     p1: Int,

compiler/testData/codegen/box/mixedNamedPosition/varargs.kt

@@ -1,4 +1,4 @@
-// !LANGUAGE: +NewInference +MixedNamedArgumentsInTheirOwnPosition
+// !LANGUAGE: +MixedNamedArgumentsInTheirOwnPosition
 
 fun foo1(
     vararg p1: Int,

compiler/testData/codegen/box/mixedNamedPosition/varargsEvaluationOrder.kt

@@ -1,4 +1,4 @@
-// !LANGUAGE: +NewInference +MixedNamedArgumentsInTheirOwnPosition
+// !LANGUAGE: +MixedNamedArgumentsInTheirOwnPosition
 // See KT-17691: Wrong argument order in resolved call with varargs. (fixed in JVM_IR)
 
 var log = ""