pi-greeter deb can't be removed if lightdm is not installed anymore. post and prerm should consider existance of lightdm.

[clemenslee]

Cheers, I moved from raspbian to devuan and wanted to remove the pi-greeter. But couldn't.
Maybe this situation should not occur normally, but somehow it did for me. Anyway, outcommenting to post installation scripts did the trick for me, so mayybe there is something
to be cleaned up. Or maybe lightdm should be a dependency and isn't. Maybe it is, but I couldn't
check now. Thanks and cheers, btw, maybe you consider moving to Devuan as a base, in the
sake of future stability... Thanks.