Remove support of 'ci_jwt_signing_key' now that it's an ApplicationSetting

doc/maintenance/index.md

files/gitlab-config-template/gitlab.rb.template

files/gitlab-cookbooks/gitlab/libraries/gitlab_rails.rb

files/gitlab-cookbooks/gitlab/recipes/gitlab-rails.rb

...lab-cookbooks/package/libraries/helpers/secrets_helper.rb

spec/chef/cookbooks/gitlab/recipes/generate_secrets_spec.rb

spec/chef/cookbooks/package/libraries/secrets_helper_spec.rb