apollo_staking: delete dead code

[asaf-sw]

Description

Remove two unused items from crates/apollo_staking/src/contract_types.rs. Both carried #[allow(dead_code)] and have zero references anywhere in the workspace.

• EPOCH_LENGTH — a pub(crate) const u64 = 100. Being pub(crate), it is not reachable outside the crate, and within apollo_staking it is never imported or referenced. The EPOCH_LENGTH names that are used elsewhere are unrelated definitions: MockStakingContract::EPOCH_LENGTH (= 30), the local const EPOCH_LENGTH in mock_staking_contract_test.rs, MIN_EPOCH_LENGTH in staking_manager.rs, and the STAKING_CURRENT_EPOCH_LENGTH metric.
• ArrayRetdata<T> — a private tuple struct (struct ArrayRetdata<T>(Vec<T>)) with zero references. It was early scaffolding (its TODO read "Remove this when we have a CairoStakingContract implementation"); the actual deserialization path uses CairoArray::try_from(retdata) in ContractStaker::from_retdata_many, so the struct was never adopted.

Evidence

• Workspace-wide grep for EPOCH_LENGTH and ArrayRetdata: the only hit for the deleted const/struct is its own definition; no use crate::contract_types::EPOCH_LENGTH/ArrayRetdata anywhere, and the test module does not reference either.
• ArrayRetdata returns 0 results across the entire starkware-industries org as well. EPOCH_LENGTH is pub(crate), so cross-repo use is impossible.
• Remaining imports in the file (CairoArray, RetdataDeserializationError, TryFromIterator) are still used by the surviving code, so no import becomes unused.

Test plan

• cargo build -p apollo_staking — passes
• cargo clippy -p apollo_staking --all-targets — clean
• SEED=0 cargo test -p apollo_staking — 91 passed, 0 failed

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Generated by Claude Code

[reviewable-StarkWare]

This change is 

[dafnamatsry]

@dafnamatsry reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on asaf-sw).