What have you tried so far? Have a look at this question. It's been a while since I did that, but I'd start with:

root@dfa-16-3e-14-27-0e:~ # openstack compute service list
+----+------------------+--------------------+----------+---------+-------+----------------------------+
| ID | Binary           | Host               | Zone     | Status  | State | Updated At                 |
+----+------------------+--------------------+----------+---------+-------+----------------------------+
|  3 | nova-scheduler   | dfa-16-3e-14-27-0e | internal | enabled | up    | 2019-03-07T08:26:12.000000 |
|  5 | nova-conductor   | dfa-16-3e-14-27-0e | internal | enabled | up    | 2019-03-07T08:26:16.000000 |
|  6 | nova-consoleauth | dfa-16-3e-14-27-0e | internal | enabled | up    | 2019-03-07T08:26:17.000000 |
|  7 | nova-compute     | dfa-16-3e-ec-08-35 | nova     | enabled | up    | 2019-03-07T08:26:12.000000 |
+----+------------------+--------------------+----------+---------+-------+----------------------------+

openstack compute service delete 7

After that I'd have a look at:

nova-manage cell_v2 list_hosts
nova-manage cell_v2 delete_host --cell_uuid <CELL_UUID> --host <HOST>

The last step should remove the host from the database. I'm not sure if more steps are required, but you can give it a try and report back. Is this a production environment or something you could break and start over?