[main] Revert multi-leader jitter

[ma2bd]

Motivation

main still runs multi-leader jitter (#6289, plus the default bump in #6359). On testnet_conway the jitter variant (#6338) was reverted in #6433 ("We found several issues in the PR") following the 2026-05-29 PM-market incident. This brings main in line by backing the feature out here too.

Alternatively we could try to fix things with #6484

Proposal

git revert of #6289 ("Multi-leader jitter"). Because main's jitter is a different commit than the one reverted on testnet_conway (#6338 was the testnet variant), this is a fresh revert rather than a cherry-pick of #6433.

Scope is jitter only:

• Removes multi_leader_jitter_target (chain client), multi_leader_proposal_delay (ownership), and the --multi-leader-jitter client option.
• Keeps the multi_leader_rounds default change (Increase default multi_leader_rounds from 2 to 5 #6359, 2 → 5) — unrelated to jitter.

One conflict in chain_client/mod.rs (the seam where #6289 inserted the jitter fn before clear_pending_proposal) was resolved by keeping main's current clear_pending_proposal definition.

Test Plan

• cargo build / cargo check clean on linera-base, linera-core, linera-client.
• cargo clippy --all-targets clean (no dangling references to removed jitter symbols anywhere in the tree).
• cargo test -p linera-base ownership green.
• CI.

Release Plan

• These changes follow the usual release cycle. (testnet_conway already reverted the equivalent change.)

Links

• Reverts: Multi-leader jitter #6289
• Mirrors the testnet_conway revert: Revert "[testnet] multi-leader jitter and conflict-detection fix (#63… #6433 (which reverted the testnet variant [testnet] multi-leader jitter and conflict-detection fix #6338)
• reviewer checklist