spec: fix destroy_session when login link is not displayed

fixes a problem that occurs when terms_of_use_spec.rb is
run immediately after a spec that hides the logout link, such as
teacher_speed_grader_spec.rb

Change-Id: I793a55b8423fcb566d55ecb3aedf1d760a90bd7d
Reviewed-on: https://gerrit.instructure.com/39104
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>