FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/simpleCall.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 o = "O"
@@ -20,4 +18,4 @@ fun B.g(a: A): String {
 
 fun box(): String {
     return B().g(A())
-}
+}