iOS doesn't have a fast way to remove multiple contacts at once. You can, however, do so on a Mac. Either open the Contacts app, or go to iCloud and select Contacts. From the list, click the contacts you want to delete while holding Command, and you can select multiples. Then, press Delete on your keyboard to remove them all at once. Because your Contacts app syncs across all devices you're signed in on with your Apple ID, the changes you make will transfer to your phone.