nginx: use UCI configuration provided by nginx-util

nginx: use UCI configuration provided by nginx-util

* update to version 1.19.6

* remove default configuration files and documentation as
  they are in the package `nginx-util`.

* do not install a `/etc/nginx/nginx.conf` file.

* use the dynamic `/etc/nginx/uci.conf` if the symlink (to
  `/var/lib/nginx/uci.conf`) is not dead after calling
  `nginx-util init_lan` (else try `/etc/nginx/nginx.conf`)

* replace nginx package by a dummy depending on `nginx-ssl`;
  the dummies will be removed after a transition period.

Signed-off-by: Peter Stadler <redacted>