FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/fromKEEP/compareTo.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 Pair<A, B>(val first: A, val second: B)
 
@@ -18,4 +16,4 @@ fun box(): String {
     return with(comparator) {
         Pair("OK", "fail").min
     }
-}
+}