Remove backend tests on old inference

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

compiler/testData/codegen/box/coroutines/kt30858.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // IGNORE_BACKEND: NATIVE
 // WITH_RUNTIME
 // WITH_COROUTINES
@@ -33,4 +32,4 @@ fun box(): String {
     m.debounce(42)
 
     return "OK"
-}
+}

compiler/testData/codegen/box/coroutines/kt31784.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // WITH_RUNTIME
 
 @file:OptIn(ExperimentalTypeInference::class)
@@ -31,4 +30,4 @@ fun box(): String {
     }
 
     return if (r == 42) "OK" else "Fail: $r"
-}
+}

compiler/testData/codegen/box/coroutines/kt35967.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // WITH_RUNTIME
 // FULL_JDK
 // ISSUE: KT-35967

compiler/testData/codegen/box/coroutines/tailCallOptimizations/unit/functionReference.kt

@@ -1,4 +1,3 @@
-// !LANGUAGE: +NewInference
 // TARGET_BACKEND: JVM
 // WITH_RUNTIME
 // WITH_COROUTINES