FIR: Unignore all blackbox tests on context receivers

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

@@ -1,7 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
-// FIR status: context receivers aren't yet supported
 
 class A {
     val ok = "OK"
@@ -14,4 +12,4 @@ class B {
 
 fun box() = with(A()) {
     B().result()
-}
+}