perf(db): drop duplicate reward disbursement index

[raymondjacobson]

Summary

• drops sol_reward_disbursements_challenge_specifier_idx concurrently when present
• keeps sol_reward_disbursements_challenge_idx, which has the same (challenge_id, specifier) key

Production evidence

• the database audit found sol_reward_disbursements_challenge_specifier_idx duplicating the checked-in sol_reward_disbursements_challenge_idx
• both indexes support the same challenge/specifier lookups, so keeping both adds write and vacuum overhead without improving read plans

Verification

• migration review only; SQL is DROP INDEX CONCURRENTLY IF EXISTS
• no application code changed