Favorite folders from external accounts are not deleted

If an INBOX of an external account is added as a favorite folder, it can no longer be deleted

Steps to reproduce:

• configure an external account
• right click on the Inbox folder and add as a favorite
• the preferred folder is added correctly
• delete the favorite folder just added: right click> remove from favorites

Expected behaviour:

• the preferred folder is deleted

Actual behaviour:

• the preferred folder is not deleted (nothing is reported in the log)