Make linkding_database_engine default to postgres when Postgres enabled

We do the same in other roles integrated into this playbook, so it's only natural.

docs/services/linkding.md

@@ -6,8 +6,8 @@
 
 This service requires the following other services:
 
--   a [Postgres](postgres.md) database, but [SQLite](https://www.sqlite.org/) is also a possibility (see `linkding_database_engine` below)
+-  a [Postgres](postgres.md) database, but will fallback to [SQLite](https://www.sqlite.org/) if unavailable
--   a [Traefik](traefik.md) reverse-proxy server
+-  a [Traefik](traefik.md) reverse-proxy server
 
 ## Configuration
 
@@ -25,13 +25,9 @@ linkding_enabled: true
 linkding_hostname: mash.example.com
 linkding_path_prefix: /linkding
 
-# We configure Linkding to use Postgres by default. See docs/services/postgres.md.
+linkding_superuser_username: ''
-# To use an external Postgres server, you need to tweak additional `linkding_database_*` variables.
+linkding_superuser_password: ''
-# Feel free to remove the line below to make Linkding use SQLite.
-linkding_database_engine: postgres
 
-linkding_superuser_username: change me
-linkding_superuser_password: change me
 ########################################################################
 #                                                                      #
 # /linkding                                                            #