FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/plusAssign.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
 
 @file:Suppress("RESERVED_VAR_PROPERTY_OF_VALUE_CLASS")
@@ -71,4 +69,4 @@ fun box(): String = with(EntityContext()) {
     v1.z = 4.0
     v1 += v0
     if (v1.x == 3.0 && v1.y == 2.0 && v1.z == 7.0) "OK" else "fail"
-}
+}