Remove R1CS_constraint_system from snarky

[mrmr1993]

This PR is the mina-side counterpart to o1-labs/snarky#674. This removes an unnecessary part of the interface.

Checklist:

• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them

[mimoo]

nice and clean!

[mrmr1993]

!ci-build-me