Remove unwanted network fm iPhone
You can't. With iOS 10.x and iOS 11.0 it's possible to delete a Wi-Fi network


if you can see it (Settings > Wi-FI)
with all other saved Wi-Fi networks using Settings > General > Reset > Reset Network Settings


You can't remove a specific Wi-Fi network otherwise.

If you have iCloud enabled you may want to sign in from another Mac and open Keychain Access.app. There you will see all Wi-Fi networks that were synced to iCloud. Delete it on the Mac with your iPhone still signed in to iCloud and restart your iPhone after about 5 minutes.