Remove secrets.yml as per the initial setup of the hmrc-manuals-api

- The initial setup of the HMRC Manuals API cited this removal as "not
  needed because the API will not be doing cookie signing." See
  https://github.com/alphagov/hmrc-manuals-api/commit/477d6500b7511a88e49615fbb48c01f48b59d754.
  It can be added back in later if it does turn out we need it here, as
  it was over in HMRC Manuals API land.
Add Airbrake for exception reporting with Errbit