build: use dpkg to purge sudo, for less spammy debug logs

build: use dpkg to purge sudo, for less spammy debug logs

apt is quite verbose:
DEBUG: buildserver > DEBUG: > sudo SUDO_FORCE_REMOVE=yes apt-get -y purge sudo
DEBUG: buildserver > Reading package lists...

DEBUG: buildserver > Building dependency tree...
DEBUG: buildserver > Reading state information...
DEBUG: buildserver > The following package was automatically installed and is no longer required:
DEBUG: buildserver >   libasprintf0c2
DEBUG: buildserver > Use 'apt-get autoremove' to remove it.
DEBUG: buildserver > The following packages will be REMOVED:
DEBUG: buildserver >   sudo*

DEBUG: buildserver > 0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
DEBUG: buildserver > After this operation, 2,391 kB disk space will be freed.
DEBUG: buildserver > (Reading database ...
(Reading database ... 5%
(Reading database ... 10%
(Reading database ... 15%
(Reading database ... 20%
(Reading database ... 25%
(Reading database ... 30%
(Reading database ... 35%
(Reading database ... 40%
(Reading database ... 45%
(Reading database ... 50%
(Reading database ... 55%
(Reading database ... 60%
(Reading database ... 65%
(Reading database ... 70%
(Reading database ... 75%
(Reading database ... 80%
(Reading database ... 85%
(Reading database ... 90%
(Reading database ... 95%
(Reading database ... 100%
(Reading database ... 73055 files and directories currently installed.)
DEBUG: buildserver > Removing sudo (1.8.10p3-1+deb8u5) ...
DEBUG: buildserver > Purging configuration files for sudo (1.8.10p3-1+deb8u5) ...
DEBUG: buildserver > Processing triggers for man-db (2.7.0.2-5) ...