Get rid of consensus_mechanism parameter

[georgeee]

We do not need consensus_mechanism parameter anymore, so this PR removes it and updates all of the code that kept using it.

Explain how you tested your changes:

• Mina compiles

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [x] Modified the current draft of release notes with details on what is completed or incomplete within this project
• [x] Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• [x] Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• [x] All tests pass (CI will check this if you didn't)
• [x] Serialized types are in stable-versioned modules
• [x] Does this close issues? None

[mergify[bot]]

⚠️ The sha of the head commit of this PR conflicts with #15642. Mergify cannot evaluate rules on this PR. ⚠️

[mrmr1993]

!ci-build-me