If you are not using on-prem anymore you can delete the arbitration mailbox by this command and uninstall Exchange from Add/Remove programs. Stop the Exchange services for couple of days to be safe before uninstall.
Get-ClientAccessServer | Set-ClientAccessServer -AutoDiscoverServiceInternalUri $Null
get-mailbox -arbitration | remove-mailbox
As a precaution before uninstalling I would turn off all exchange services and verify if you have any issues before hand.