Don't set DATABASE_HOST by default

This allows the application to connect to the database over a Unix
socket, and is also no longer needed by CI.

Jira-Issue: MAV-6379
Jira-Issue: MAV-6706

Author: thomasleese

config/database.yml

@@ -18,8 +18,7 @@ end_to_end:
 test:
   <<: *default
   database: manage_vaccinations_test<%= ENV['TEST_ENV_NUMBER'] %>
-  # Needed by CI server
-  host: <%= ENV.fetch("DATABASE_HOST", "localhost") %>
+  host: <%= ENV.fetch("DATABASE_HOST", "") %>
   username: <%= ENV.fetch("DATABASE_USER", "") %>
   password: <%= ENV.fetch("DATABASE_PASSWORD", "") %>
 staging: