Sometimes you would like to uninstall a mailbox server or un-install a mailbox server but when you make the Get-Mailbox -Arbitration -Database DataBase1 you see some mailboxes there. These are there to prevent you from deleting the mailbox database or server. These are used by the Exchange server. You can either move or delete these mailboxes. If this is not the last mailbox server in your setup I would suggest you move them, otherwise delete them or you will not be able to uninstall the server.

Moving the mailboxes
Get-Mailbox -Arbitration -Database EXCHDB1| New-MoveRequest -TargetDatabase EXCHDB2

Disabling the mailboxes
Get-Mailbox -Arbitration -Database EXCHDB1| Disable-Mailbox -Arbitration

Deleting the mailboxes
Get-Mailbox -Arbitration -Database EXCHDB1| Remove-Mailbox -Arbitration -RemoveLastArbitrationMailboxAllowed

If you would like to re-create them, I would suggest the below

Setup.com /PrepareAD /IAcceptExchangeServerLicenseTerms
or
.\Setup /PrepareAD /IAcceptExchangeServerLicenseTerms