How to delete Google account from a factory-resetted phone?
(I've done reset via some hardware menu, it had such items as wipe device, etc.)

After asking the question:

I've logged into Google Accounts as that user on a desktop, and removed that device from a list of "my devices"

It did not change anything. 

Then I've logged on THE DEVICE as that user. Then I was provided an option to login as a different Google account, as I did. So that solved my problem.