chore(backend): Remove signing apis

[bitdivine]

Motivation

The backend canister signing APIs are now provided by the chain fusion signer.

Changes

• Remove all signing and threshold signature APIs from the backend canister.
• Remove the utilities and tests for these methods.
• Fixes:
  • Get the SignRequest typescript data type from the signer bindings rather than the backend bindings. This was an oversight but has no consequence as the types are the same.

Not removed:

• Anything beyond the bulk removal of methods and immediate dependencies. For example:
  • The migration methods, or if we wish to keep the migration methods, the threshold signing aspects of migration.
  • Removal of dependencies in Crates.toml that may be used only for signing.
  • I have not made a detailed scan for other small things that might be removed or simplified.
  • Any frontend simplification possible now that the backend and signer are definitely distinct.

Tests

• Existing CI should cover the remaining functionality.
• The chain fusion signer CI should cover the signing methods comprehensively.

[AntonioVentilii-DFINITY]

LGTM, tks!