[IR] Remove JS-specific manglers, as they don't customize any behavior

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/mangle/descriptor/DescriptorMangleComputer.kt

@@ -18,7 +18,7 @@ import org.jetbrains.kotlin.types.typeUtil.isUnit
 /**
  * The descriptor-based mangle computer. Used to compute a mangled name for a declaration given its [DeclarationDescriptor].
  */
-abstract class DescriptorMangleComputer(builder: StringBuilder, mode: MangleMode) :
+open class DescriptorMangleComputer(builder: StringBuilder, mode: MangleMode) :
     BaseKotlinMangleComputer<
             /*Declaration=*/DeclarationDescriptor,
             /*Type=*/KotlinType,
@@ -30,7 +30,7 @@ abstract class DescriptorMangleComputer(builder: StringBuilder, mode: MangleMode
             >(builder, mode) {
     final override fun getTypeSystemContext(session: Nothing?): TypeSystemContext = SimpleClassicTypeSystemContext
 
-    abstract override fun copy(newMode: MangleMode): DescriptorMangleComputer
+    override fun copy(newMode: MangleMode) = DescriptorMangleComputer(builder, newMode)
 
     final override fun DeclarationDescriptor.visitParent() {
         containingDeclaration?.visit()

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

@@ -19,7 +19,7 @@ import org.jetbrains.kotlin.ir.visitors.acceptVoid
 /**
  * A mangle computer that generates a mangled name for a Kotlin declaration represented by [IrDeclaration].
  */
-abstract class IrMangleComputer(
+open class IrMangleComputer(
     builder: StringBuilder,
     mode: MangleMode,
     protected val compatibleMode: Boolean
@@ -40,7 +40,7 @@ abstract class IrMangleComputer(
 
     open fun IrFunction.platformSpecificFunctionMarks(): List<String> = emptyList()
 
-    abstract override fun copy(newMode: MangleMode): IrMangleComputer
+    override fun copy(newMode: MangleMode) = IrMangleComputer(builder, newMode, compatibleMode)
 
     final override fun IrDeclaration.visitParent() {
         parent.acceptVoid(Visitor())