remove /usr/local from package

https://lintian.debian.org/tags/file-in-usr-local.html

Instead the following commands are installed via debian/links:
/usr/bin/mintsources
/usr/bin/software-properties-gtk
/usr/bin/software-properties-kde
/usr/bin/apt-add-repository

And add-apt-repository will be installed into /usr/local/bin as a link to /usr/bin/add-apt-repository-mint via maintainer script.