Remove EpicsController#index.json endpoint

The following discussion from !15110 (merged) should be addressed:

• @engwan started a discussion: (+4 comments)

  Thanks @felipe_artur! Code looks good, just a suggestion on the param description.

  Also, I think we don't have to wait for frontend here and just have the changes to make the frontend actually use this in a separate MR.

  When all this is done, I think we should also create a tech debt issue to remove the current endpoint.

The current endpoint is EpicsController#index.json; we don't need this any more.