[JS IR BE] Remove support for dynamic symbols and descriptors

Dynamic expressions in IR are now represented as IrDynamicOperatorExpression

We don't have dynamic functions, dynamic dispatch receivers anymore

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -424,8 +424,6 @@ fun IrDeclaration.isEffectivelyExternal(): Boolean {
     }
 }
 
-fun IrDeclaration.isDynamic() = this is IrFunction && dispatchReceiverParameter?.type is IrDynamicType
-
 inline fun <reified T : IrDeclaration> IrDeclarationContainer.findDeclaration(predicate: (T) -> Boolean): T? =
     declarations.find { it is T && predicate(it) } as? T