FIR: Unignore all blackbox tests on context receivers

compiler/testData/codegen/box/extensionFunctions/contextReceivers/plusMatrix.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
 
 interface NumberOperations {
@@ -50,4 +48,4 @@ fun box(): String {
     with(DoubleOperations) {
         return if (m1 + m2 == matrixOf(2, 2, 1.4, 2.3, 3.2, 4.1)) "OK" else "fail"
     }
-}
+}