FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/fromKEEP/dp.kt

@@ -1,7 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
-// FIR status: context receivers aren't yet supported
 // WITH_STDLIB
 
 class View {
@@ -17,4 +15,4 @@ fun box(): String {
         }
         return "fail"
     }
-}
+}