FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/arrayAccessOperators.kt

@@ -1,7 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
-// FIR status: context receivers aren't yet supported
 
 data class MyContainer(var s: String)
 
@@ -22,4 +20,4 @@ fun box(): String {
         myContainer[0] = "OK"
         myContainer[0] ?: "fail"
     }
-}
+}