Remove Rails override

The config already has `update_external_dependencies: false`, so Rails updates are already blocked.

.govuk_dependabot_merger.yml

@@ -2,6 +2,3 @@ api_version: 2
 defaults:
   auto_merge: true
   update_external_dependencies: false # TODO: enable after verifying that this repo meets the conditions
-overrides:
-  - dependency: rails # should be upgraded manually, see https://docs.publishing.service.gov.uk/manual/keeping-software-current.html#rails
-    auto_merge: false