Drop all tests that disable features for language version less than 3

compiler/testData/diagnostics/tests/controlFlowAnalysis/unusedInAnonymous.kt

@@ -1,10 +0,0 @@
-// FIR_IDENTICAL
-// !LANGUAGE: -SingleUnderscoreForParameterName
-// See KT-8813, KT-9631
-
-fun someApi(f: (Int) -> Unit) = f(42)
-
-fun test() {
-    someApi(fun(p: Int) {})
-    // Apparently "p" cannot be removed because the signature is fixed by "someApi
-}

compiler/testData/diagnostics/tests/controlFlowAnalysis/unusedInAnonymous.txt

@@ -1,4 +0,0 @@
-package
-
-public fun someApi(/*0*/ f: (kotlin.Int) -> kotlin.Unit): kotlin.Unit
-public fun test(): kotlin.Unit