Remove possibly unused key

The following discussion from !72212 (merged) should be addressed:

• @krasio started a discussion:

  Is this one used? I find it mentioned only here and in tests:

  $ git grep expires_at_formatted
  app/controllers/concerns/membership_actions.rb:                      expires_at_formatted: member.expires_at.to_time.in_time_zone.to_s(:medium)
  spec/controllers/groups/group_members_controller_spec.rb:            "expires_at_formatted" => expiry_date.to_time.in_time_zone.to_s(:medium)
  spec/controllers/projects/project_members_controller_spec.rb:            "expires_at_formatted" => expiry_date.to_time.in_time_zone.to_s(:medium)