sqldb: drop redundant and unused invoice indexes

Remove four indexes from the invoices table that either duplicate existing
UNIQUE constraint indexes or are never used in WHERE clauses:

- invoices_hash_idx: redundant, UNIQUE constraint on hash already creates
  an implicit index
- invoices_payment_addr_idx: redundant, UNIQUE constraint on payment_addr
  already creates an implicit index
- invoices_preimage_idx: unused, preimage is NULL on all new invoices and
  is never used as a query filter
- invoices_settled_at_idx: unused, settled_at is NULL on all pending
  invoices and is never used as a query filter (settle_index is used
  instead)

Dropping these reduces B-tree working set size, which improves page cache
utilization as the invoice table grows.

sqldb/sqlc/migrations/000012_drop_redundant_invoice_indexes.down.sql

@@ -0,0 +1,4 @@
+CREATE INDEX IF NOT EXISTS invoices_hash_idx ON invoices(hash);
+CREATE INDEX IF NOT EXISTS invoices_payment_addr_idx ON invoices(payment_addr);
+CREATE INDEX IF NOT EXISTS invoices_preimage_idx ON invoices(preimage);
+CREATE INDEX IF NOT EXISTS invoices_settled_at_idx ON invoices(settled_at);