Remove some unused code in snark-related modules

[georgeee]

Removes some code which was unused:

• Function Transaction_state.verify
• Function Blockchain_state.verify
• Function to_input and module Checked from Snarked_ledger_state.Statement.With_sok

Code was written but never used. More importantly, implementation of Snarked_ledger_state.Statement.With_sok.to_input (and transitively Transaction_state.verify) didn't agree with verify function exposed by Proof module (which used to_field_elements derived from typ instead of to_input).

PR removes a function that was unused before #16201, where it was used and the new code become unexpectedly incorrect.

Explain how you tested your changes:

• [x] Code compiles

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [ ] 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