Fix make uninstall

[tuurep]

Hey, turns out I found some issues in the Makefile, and the fix seems perfect :smile:

Please make sure for example that /usr/lib/web-greeter/ is removed as intended and in the right place.

Plus added a small correction for \ns being literal in make uninstall_preserve output as follows:

 Themes are not uninstalled. Remove them manually or use `make uninstall_all`:	\n/usr/share/web-greeter
 web-greeter config was not uninstalled. Remove it manually or use `make uninstall_all`:	\n/etc/lightdm/web-greeter.yml

This PR should especially make it easier to switch between a manual build and building with AUR.