FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/fromKEEP/monoidSum.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
 
 interface Semigroup<T> {
@@ -30,4 +28,4 @@ fun box(): String {
     return with(StringMonoid) {
         listOf("O", "K").sum()
     }
-}
+}