remove facebook_test_app and update config example (#419)

we only used facebook_test_app in one test, we weren't declaring it in
our config file. Besides, since we can specify the values by environment
I think the different variable name is not needed.

config/config.yml.example

@@ -127,6 +127,7 @@ test:
   storage_video_bucket:
   storage_video_asset_path:
   storage_medias_asset_path: 'http://localhost:9000/check-test/medias'
+  facebook_app: # '<FACEBOOK APP ID>:<FACEBOOK APP SECRET>'
   otel_log_level: off
   otel_traces_sampler:
   sentry_dsn:

test/models/metrics_test.rb

@@ -5,7 +5,7 @@ class AllTestFacebookAppsRateLimited < StandardError; end
 
   test "should get metrics from Facebook" do
     begin
-      fb_config = PenderConfig.get('facebook_test_app') || PenderConfig.get('facebook_app')
+      fb_config = PenderConfig.get('facebook_app')
       PenderConfig.current = nil
       key = create_api_key application_settings: { config: { facebook_app: fb_config }}