Hi Alexey,
You can also delete the Follower from the CLI (On Leader) as below:
SZ-100Leader# show cluster-state (to get the Follower node name)
SZ-100Leader# config
SZ-100Leader(config)# no control-plane
I haven't checked this in 3.5.1 version, but tested on later releases.
Regards,
Sanjay Kumar