Pokemon Go iPhone - deleting accounts from sign-in screen

I have an iPhone and have the max number of accounts (10) - I want to delete 3 of these, as they are friends that have used my phone previously but no longer play.

When I try to add another account, it tells me:

"You're signed in to the maximum number of accounts. To use another account, sign out of all Google Accounts on this device."

Since no one is signed in to these accounts, it seems like they are already "signed out" but that doesn't seem to be the case. I have tried numerous things and can't figure out how to get these other accounts disconnected from my device.

Install Gmail.

It gives the option to remove a login from the account settings page in Gmail iOS on either iPhone or iPad. To remove a login, tap Accounts > Edit > Remove for all the logins you want to remove. Removing a Google login from one app deletes it for all installed Google apps on the device.

If you happen to have these on your phone already, you could also try the Google Search App or even the YouTube app to remove the account(s).

Similar problems and solution can be found in this thread also: https://www.reddit.com/r/TheSilphRoad/comments/6g13eb/how_to_remove_pokemon_go_account_login_option/