BIG-IQ "Status of HA Certificate on this device has changed from EXPIRING SOON to EXPIRED"

This sounds like a device certificate. Take a look here, as you may need to use the CLI to accomplish this:

https://techdocs.f5.com/en-us/bigiq-8-0-0/big-iq-web-application-security/configure-connection-device-secure-communications.html

Generally, you would not remove a device certificate, but instead update it. Classically, in BIG-IP, old software came with a low key bitsize or short expiration window certificate. It also was generic. Even if you dont plan on using an internal CA or removing cert warnings, still best practive to replace with a large key size certificate, an expiration taking into account how long you plan to keep the product before disposal or replacement, and use a common name equivalent to the shortname of the FQDN give to the device (so if the company gets sold, or is renamed, you just need to rename the host to the new domain and can keep the old device name without regenerating a new cert).

Hope this helps, and this is indeed a device certificate.