FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/overloading.kt

@@ -1,7 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
-// FIR status: context receivers aren't yet supported
 
 context(Int, String)
 fun foo(): String = "O"
@@ -19,4 +17,4 @@ fun box(): String {
         foo()
     }
     return o + k
-}
+}