Revert "Prevent some unwanted packages to be installed at all, rather than...

Revert "Prevent some unwanted packages to be installed at all, rather than uninstalling them later."

This reverts commit 87a2c027.

Conflicts:
	config/chroot_apt/preferences

Setting the pin-priority of some packages to -1, make it impossible to
install them with a canonical `apt-get install $package` command.