The command cannot be used:
Set-MailboxDatabase MB-BR-01 -PublicFolderDatabase $ null
as it returns an error, it does not allow the public folder associated with the DB to be null
but you can use the following system:
Navigate to the container that contains the Exchange databases. For Exchange 2010 you will find this in CN = Services -> CN = Microsoft Exchange -> CN = (the organization name) -> CN = Administrative Groups -> CN = Exchange administrative group (FYDIBOHF23SPDLT) -> CN = Database.
Right-click the mailbox database that you want to remove the default public folder database from and select Properties.
Scroll down until you find the msExchHomePublicMDB attribute. Highlight it and then clickEdit.
Click the Clear button so that the value changes to “not set”.
Click OK and OK again to confirm the change.
Depending on the size of the network, you may have to wait for that change to replicate throughout the Active Directory.
When you try to remove the public folder database again you should find that it now removes without errors.