Remove deprecated continuous onboarding route

The following discussion from !109243 (merged) should be addressed:

• @dstull started a discussion:

  Deprecated route - remove on next milestone

• logs to watch for old path: https://log.gprd.gitlab.net/goto/5fede250-aee6-11ed-9f43-e3784d7fe3ca

• logs to watch for hitting the new path: https://log.gprd.gitlab.net/goto/af9d0dd0-aee6-11ed-85ed-e7557b0a598c