Y24-523 - Remove Heron tube racks code

[KatyTaylor]

Describe the Housekeeping
There is legacy code in Sequencescape to support the creation of tube racks through the API, for the Heron project. This was used for an early iteration of the Heron pipeline, when samples were being processed in the cellular labs and received into SeqOps in tube racks. As far as I know, the code that calls the endpoint has already been deleted (wrangler app, which was polling a table in MLWH?).

• Remove obsolete Heron tube rack endpoint and associated code.

Additional context
Take a look at:

• routes - heron namespace - tube_racks
• controllers/api/v2/heron
• models/heron/factories/tube_rack
• 'concerns' used within above factory

This was noticed as part of the work to improve SS & Limber's support for tube racks (stories coming out of sanger/limber#1482).

A lot of the above code is fairly generic, despite being used for Heron only. It could be useful if the same person who does this story also does this refactoring story, in case there is useful code that could be salvaged.