Correct remaining `Lint/UselessAssignment` haml-lint violations in EE

ee/app/views/projects/mirrors/pull/_authentication_method.html.haml

 - import_data = f.object
 - regen_data = { auth_method: 'ssh_public_key', regenerate_ssh_private_key: true }
-- ssh_key_auth = import_data.ssh_key_auth?
 - ssh_public_key_present = import_data.ssh_public_key.present?
 
 .form-group

ee/app/views/projects/services/gitlab_slack_application/_help.html.haml

 - pretty_name = defined?(@project) ? @project.full_name : 'namespace / path'
-- run_actions_text = "Perform common operations on GitLab project: #{pretty_name}"
 
 .card
   %p

ee/app/views/shared/billings/_billing_plan_header.html.haml

@@ -23,5 +23,4 @@
     = render 'shared/billings/trial_status', namespace: namespace
 
   - if namespace.eligible_for_trial?
-    - namespace_billing_url = namespace.is_a?(Group) ? group_billings_url(namespace) : profile_billings_url
     %p= link_to 'Start your free trial', new_gitlab_com_trial_url, class: 'btn btn-primary btn-inverted'

ee/app/views/shared/issuable/_assignee_page_default.html.haml

-- filter_placeholder = local_assigns.fetch(:filter_placeholder, 'Search')
-
 .dropdown-page-one
   .issue-board-dropdown-content
     %p

ee/app/views/shared/milestones/_burndown.html.haml

 - milestone = local_assigns[:milestone]
-- project = local_assigns[:project]
 - burndown = burndown_chart(milestone)
 - warning = data_warning_for(burndown)