FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionClasses/edouble.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 EntityContext {
     var d = DoubleArray(16)
@@ -21,4 +19,4 @@ fun box(): String {
         eDouble.value = .2
         return if (eDouble.value == .2) "OK" else "fail"
     }
-}
+}