Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/controlStructures/kt2598.kt

@@ -0,0 +1,9 @@
+fun foo(condition: Boolean): String {
+    val u = if (condition) {
+        "OK"
+    } else {
+    }
+    return u.toString()
+}
+
+fun box() = foo(true)