Proper removal of /etc/linuxmint/adjustments/10-mintsystem-synaptic.overwrite

This ensures that the goddamn file will be really removed on the package upgrade, not just left there and marked as obsolete (like dpkg does with the files from /etc by default).
That means less confusion for (curious) users.

Used sources:
* [Debian wiki - dpkg conffile handling](https://wiki.debian.org/DpkgConffileHandling)
* [Raphaël Hertzog - The right way to remove an obsolete conffile in a Debian package](http://raphaelhertzog.com/2010/10/07/the-right-way-to-remove-an-obsolete-conffile-in-a-debian-package/)
* [man dpkg-maintscript-helper](http://man7.org/linux/man-pages/man1/dpkg-maintscript-helper.1.html)
* Googling "debian/maintscript" for the examples of usage