Remove check for files count for mirror elements set in light classes

It is workaround about intensive exception #EA-121413. There is no idea how to fix it fast but it is a try. In case of using UltraLight classes we hope not to
use old fashion light classes anymore soon. (Fixed #KT-28193)

compiler/light-classes/src/org/jetbrains/kotlin/asJava/builder/LightClassBuilder.kt

@@ -98,14 +98,6 @@ private fun createJavaFileStub(project: Project, packageFqName: FqName, files: C
             super.appendMirrorText(indentLevel, buffer)
         }
 
-
-        override fun setMirror(element: TreeElement) {
-            if (files.size == 1) {
-                LOG.error("Mirror element should never be set for light classes generated from a single file")
-            }
-            super.setMirror(element)
-        }
-
         override fun getMirror(): PsiElement {
             if (files.size == 1) {
                 LOG.error("Mirror element should never be calculated for light classes generated from a single file")