Packaging is broken for debian-based systems after removing
xtrabackup scripts. This is due to the fact that links to the
scripts are not removed from the installation file
(from the debian/mariadb-server-10.3.install).

Also in this fix some comments have been added to the
documentation, indicating that using xtrabackup[-v2] is
an deprecated, therefore user should use mariabackup instead.

https://jira.mariadb.org/browse/MDEV-18064