I would like to delete selected backups in the Backup Manager from the command line. Is there a plesk command to delete backups from the Backup Manager?
No, there is no such command. If you want to delete a particular backup from the server, you can do it from linux shell.For example,
rm -f /var/lib/psa/dumps/my_backup_1504280530.xml
rm -rf /var/lib/psa/dumps/.discovered/my_backup_1504280530