Remove allow list from Dependabot config

In [RFC-153](alphagov/govuk-rfcs#153), we proposed and agreed to remove 'allow lists' from all [GOV.UK](http://gov.uk/) Dependabot configs, in order to re-enable security updates.

Trello card: https://trello.com/c/DuA0q9Ck/2966-remove-allow-lists-from-dependabot-configs-2

.github/dependabot.yml

@@ -4,30 +4,7 @@ updates:
     directory: /
     schedule:
       interval: daily
-    allow:
-      # Security updates
-      - dependency-name: brakeman
-        dependency-type: direct
-      # Internal gems
-      - dependency-name: "govuk*"
-        dependency-type: direct
-      - dependency-name: "rubocop-govuk"
-        dependency-type: direct
-      - dependency-name: gds-api-adapters
-        dependency-type: direct
-      - dependency-name: gds-sso
-        dependency-type: direct
-      - dependency-name: plek
-        dependency-type: direct
-      # Framework gems
-      - dependency-name: mongo
-        dependency-type: direct
-      - dependency-name: factory_bot_rails
-        dependency-type: direct
-      - dependency-name: rails
-        dependency-type: direct
-      - dependency-name: rspec-rails
-        dependency-type: direct
+
   - package-ecosystem: docker
     directory: /
     schedule: