FIR: Unignore all blackbox tests on context receivers

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

@@ -1,7 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
-// FIR status: context receivers aren't yet supported
 
 interface Canvas {
     val suffix: String
@@ -21,4 +19,4 @@ object MyCanvas : Canvas {
     override val suffix = ""
 }
 
-fun box() = with(MyCanvas) { Circle().draw() }
+fun box() = with(MyCanvas) { Circle().draw() }