Remove backend tests on old inference

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

compiler/testData/codegen/box/delegatedProperty/kt35707.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // ISSUE: KT-35707
 
 import kotlin.reflect.KProperty
@@ -17,4 +16,4 @@ fun box(): String {
     } else {
         "FAIL"
     }
-}
+}

compiler/testData/codegen/box/delegatedProperty/optimizedDelegatedProperties/mixedArgumentSizes.kt

@@ -1,5 +1,3 @@
-// !LANGUAGE: -NewInference
-
 inline operator fun Double.provideDelegate(thisRef: Any?, kProp: Any?) = this.toLong()
 
 inline operator fun Long.getValue(thisRef: Any?, kProp: Any?) = this.toInt()

compiler/testData/codegen/box/delegatedProperty/provideDelegate/genericProvideDelegateOnNumberLiteral.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // WITH_RUNTIME
 
 import kotlin.properties.ReadOnlyProperty
@@ -26,4 +25,4 @@ fun box(): String {
     if (byStringNullable != "strNullable") return "fail5"
 
     return "OK"
-}
+}