packages: remove auto-installed on removal

i've been noticing that in the binary_syslinux stage a wholebunch of packages get installed whilst installing isolinux, and then upon removing the installed packages at the end of the script, apt is listing a large number of those that were auto installed as no longer in use and can be removed via apt autoremove. this then persists through package installation and removal steps throughout remaining scripts.

adding --auto-remove to apt-get remove --purge and similarly --purge-unused to aptitude purge instructs apt to get rid of the unused auto-installed packages at the same time as removing specific package that brought them in in the first place, and thus resolves the problem.

Gbp-Dch: Short