Set experimentation cookie for GitLab domain only

Previously the cookie experimentation_subject_id was set for the top
domain, even when GitLab was installed on a subdomain. This commit
changes that so that the cookie is only installed for the GitLab domain.
This brings it in line with all the other cookies that GitLab sets.

changelogs/unreleased/experimentation-cookie-domain.yml

+---
+title: Set experiementation cookie for GitLab domain only
+merge_request:
+author:
+type: fixed

lib/gitlab/experimentation.rb

@@ -57,7 +57,6 @@ module Gitlab
 
         cookies.permanent.signed[:experimentation_subject_id] = {
           value: SecureRandom.uuid,
-          domain: :all,
           secure: ::Gitlab.config.gitlab.https,
           httponly: true
         }