Can't remove a folder created with doveadm import from IMAP client

Hi,

I imported mails with the doveadm import command. It worked great, but now, I'm unable to remove the folder backup-2019-03-22? In Thunderbird, the folder is displayed with grey italic font.

The command I ran: doveadm import -u user@example.com mdbox:/var/vmail/domains/ example.com/user/mdbox-backup INBOX/backup-2019-03-22 all

I found this help page from Mozilla: http://kb.mozillazine.org/Grey_italic_folders. It says Thunderbird display the folder in grey italic when the folder have the \Noselect flag. Is it the case with the problem I'm having? If yes, how to remove that flag on the folder?

Thanks.

-- Francis