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

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