DAL/RPC: remove GET /commitments/<c>/proof

What

Stacked on !13075 (merged)

Remove the low-level RPC that was used before the existence of POST /slots to obtain the commitment proof before commitment publication.

Why

It is not useful anymore and we want to avoid indexing RPCs by commitments.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR