Remove dead acquireAdvisoryXactLock try variant

The transaction-scoped TRY variant had no production callers — only the
conn-level try-lock (acquireAdvisoryLock) and the blocking xact variant
(acquireAdvisoryXactLockBlocking, used by the admin cancellation path) are in
use. Dropping the dead code removes a foot-gun: a try-lock that silently
fails to acquire inside a transaction would otherwise look like a safe option.

backend/handlers/payments/locks.go

@@ -39,15 +39,8 @@ func acquireAdvisoryLock(ctx context.Context, conn *pgxpool.Conn, key string) (b
 	return tryAdvisoryLock(ctx, conn, key, "pg_try_advisory_lock")
 }
 
-// acquireAdvisoryXactLock is the transaction-scoped variant of
-// acquireAdvisoryLock: the lock is held on the transaction and auto-released
-// at commit/rollback, so the caller must NOT unlock explicitly.
-func acquireAdvisoryXactLock(ctx context.Context, tx pgx.Tx, key string) (bool, error) {
-	return tryAdvisoryLock(ctx, tx, key, "pg_try_advisory_xact_lock")
-}
-
 // acquireAdvisoryXactLockBlocking is the transaction-scoped BLOCKING variant
-// of acquireAdvisoryXactLock: it issues `SELECT pg_advisory_xact_lock(...)`
+// of acquireAdvisoryLock: it issues `SELECT pg_advisory_xact_lock(...)`
 // ONCE and waits for as long as the key is contended — there is no 3s bound.
 // The lock is transaction-scoped, so it is auto-released at the caller's
 // commit/rollback (never unlocked explicitly).