Remove tvheadend user on purge

This fixes tvheadend#1722 on my test system.

debian/tvheadend.postrm

@@ -9,6 +9,7 @@ case "$1" in
 purge)
     if getent passwd "$HTS_USER" >/dev/null; then
         rm -rf "$(getent passwd "$HTS_USER" | cut -d':' -f6)"
+        deluser --system "$HTS_USER"
     fi
     db_purge
    ;;