Remove set_validator_weight admin override

[LandynDev]

Why

set_validator_weight was an admin-only instruction that let the admin unilaterally rewrite any validator's lottery draw weight. That is an unnecessary centralization / skew vector on the reservation lottery. The legitimate weight surface is the consensus path (vote_set_weights), with add_validator(key, weight) setting the initial value at whitelist time. Decision (user, 2026-06-29): delete the override.

What's removed

• instructions/admin.rs: the set_validator_weight handler.
• lib.rs: its #[program] dispatch entry.
• Its test coverage (the two set-weight LiteSVM cases in tests/test_validator_weight.rs; the onchain_set_validator_weight integration case + its set_validator_weight_ix builder in tests/integration_onchain.rs).

No event or error was defined solely for it — the handler reused ErrorCode::ValidatorNotFound (shared with remove_validator), which stays.

What's kept (the legitimate weight surface)

• add_validator(key, weight) — sets a validator's initial draw weight.
• vote_set_weights — consensus-governed weight vector (the normal path); full coverage retained.
• ValidatorInfo.weight field + the Phase-9 lottery draw that consumes it — unchanged.

Notes

• Pure instruction-surface removal. Anchor discriminators are per-instruction name-hashes, so removing one does not shift the others; all kept instructions' discriminators/layouts are unchanged.
• Config schema is unchanged (no field added/removed), so CONFIG_VERSION is not bumped — it tracks the Config account layout, not the instruction surface.
• Repo-wide grep -rn set_validator_weight returns nothing (contract + off-chain python/ts/json). No off-chain builder/layout referenced this instruction.
• anchor build clean; full LiteSVM unit suite green (test_validator_weight now 6 tests: add-with-weight + 5 vote_set_weights cases).

CONTRACT CHANGE — please run a smart-contract-security review before merge.