Remove documentation on confirming plugins work

** Why are these changes being introduced:

* The how-to page on confirming whether all of our various plugins are
  working is something that the UX group has indicated an interest in
  helping to maintain, and the testing itself will be followed by both
  engineers and site builders.

  Given this, it makes sense to move this documentation to the service
  documentation in Confluence.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/lm-60

** How does this address that need:

* This removes the stub page on confirming plugin operation, as the
  content has been moved to the Confluence space. This isn't a link that
  needs to be maintained in this way.

** Document any side effects to this change:

* None

Author: matt-bernhardt