This Mailbox database contains one or more mailboxes, mailbox plans, archive mailboxes, public folder mailboxes or arbitration mailboxes. ...
With Exchange 2013 you were able to remove the database after moving any regular user mailboxes
Get-Mailbox -Database
<dbname> | New-MoveRequest –TargetDatabase <dbname>
then moving the arbitration mailbox
Get-Mailbox -Database
<dbname> -Arbitration | New-MoveRequest –TargetDatabase <dbname>
but now with Exchange 2016 there is a new type of mailbox know as the "AuditLog" mailbox. This is a change in the architecture of mailbox auditing which no longer stores audit data on the individual mailboxes but rather stores the data in central audit mailboxes. This provides better long term storage as well as better reporting and exporting of data. The structure matches that of Office 365 to also help keep things consistent with hybrid environments. So to remove that AuditLog mailbox just use the new switch to move the mailbox:
Get-Mailbox -Database
<dbname> -AuditLog | New-MoveRequest –TargetDatabase <dbname>
Once all the mailboxes have been moved off you should now be able to remove the default database.