I had previously linked a Nextcloud account somehow (possibly through Online Accounts in Gnome’s Control Center) so that it was present in Nautilus, and there was no GUI means to remove it.
What I did to finally make it go away was to first open Seahorse and the Login keyring therein, and delete all keys related to the account. I then deleted ~/.config/goa-1.0/accounts.conf.
My accounts.conf only had the unwanted account in it, so you need to edit it instead of deleting it if yours has other accounts that you wish to preserve.
Deleting the keys (in Seahorse) may not be necessary, but didn’t hurt either.