Merge boxMultiFile testData into box, delete BoxMultiFile test

compiler/testData/codegen/box/reflection/mapping/topLevelFunctionOtherFile.kt

@@ -5,6 +5,7 @@ fun test2() {
 }
 
 // FILE: main.kt
+// See KT-10690 Exception in kotlin.reflect when trying to get kotlinFunction from javaMethod
 
 import kotlin.reflect.jvm.javaMethod
 import kotlin.reflect.jvm.kotlinFunction

compiler/testData/codegen/box/regressions/internalTopLevelOtherPackage.kt

@@ -6,4 +6,4 @@ fun box() = a.x
 
 package a
 
-val x: String = "OK"
+internal val x: String = "OK"

compiler/testData/codegen/boxMultiFile/sameFileName.kt

@@ -1,7 +0,0 @@
-// FILE: 1/box.kt
-
-fun box() = box1() + "K"
-
-// FILE: 2/box2.kt
-
-fun box1() = "O"