[JS IR] Clean up Declaration Table code

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DeclarationTable.kt

@@ -26,7 +26,6 @@ abstract class DeclarationTable(val builtIns: IrBuiltIns, val descriptorTable: D
     private val table = mutableMapOf<IrDeclaration, UniqId>()
     val descriptors = descriptorTable
     protected abstract var currentIndex: Long
-    protected abstract val initialIndex: Long
 
     open fun loadKnownBuiltins(): Long {
         builtIns.knownBuiltins.forEach {

compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsDeclarationTable.kt

@@ -15,5 +15,6 @@ class JsDeclarationTable(builtIns: IrBuiltIns, descriptorTable: DescriptorTable)
 
     init {
         loadKnownBuiltins()
+        currentIndex = 0
     }
 }