FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/passingLambdaToContextualParam.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 C {
     val result = "OK"
@@ -9,4 +7,4 @@ class C {
 
 fun contextual(f: context(C) () -> String) = f(C())
 
-fun box(): String = contextual { result }
+fun box(): String = contextual { result }