fix: incorrect index declaration

[suyuan32]

No description provided.

[hsluoyz]

Thanks for catching this — the declaration really was dead code, and the
generated schema confirmed the index had never existed.

I've landed the fix in ec45d2b rather than merging
this PR, because the rename alone doesn't hold up on MySQL: seven utf8mb4
varchar(255) columns need 7140 bytes of index key against InnoDB's 3072-byte
limit, which is the Error 1071 the CI run here hit. That commit narrows the
policy columns to varchar(100) (matching casbin's canonical schema and
gorm-adapter), and makes inserts ignore conflicts so that re-adding an
already-stored rule stays a no-op instead of becoming an error for replicas
whose in-memory model has fallen behind. The README now documents the
duplicate-row and over-long-value checks needed before upgrading an existing
database.

Closing in favour of that commit — but the diagnosis was yours, and it was a
good catch.