refactor(provider): remove residual ADR-017 plumbing

[giunatale]

Closes #60

ADR-017 (inactive provider validators) was already declared removed in DESIGN_RATIONALE.md, but its MaxProviderConsensusValidators cap and the ComputeNextValidators machinery were left behind. The cap was inert — the default (180) is never below staking MaxValidators, so it never truncated anything — yet the docs still described the consumer set as "capped".

This finishes the removal:

• Deletes the max_provider_consensus_validators param (incl. proto) and the
  GetMaxProviderConsensusValidators getter.
• Deletes ComputeNextValidators / GetLastProviderConsensusActiveValidators;
  the consumer set is now the full bonded set (relay + genesis iterate it
  directly), and the provider consensus set follows staking MaxValidators.
• Removes the two cap-only invariants and the capped staking-keeper shim
  (never wired into mint/distribution — the app injects StakingKeeper).
• Updates the docs to describe the uncapped, full-set behavior.

[tbruyelle]

Nice cleaning, a few non-blocking comments.