Remove non-spec description field from lexPermissionSet schema

Removed a description field from the lexPermissionSet Zod schema that was not defined in the Lexicon permission-set specification.

What

The lexPermissionSet schema in the Zod validator had a description field that does not exist in the official Lexicon spec. I removed it and verified the change against the spec document, the SDK options type, and all permission-set JSON files in the repo. All lexicon tests passed and the build succeeded.

Why

The schema should match the spec exactly. Extra fields in the validator can cause confusion about what is actually required and make it harder to catch real spec violations. Keeping the schema aligned with the spec is simpler to maintain.

Takeaway

Schema validators should only include fields that are in the spec. Removing unnecessary fields reduces noise and makes the validator more useful for catching actual problems.

Draft PR: https://github.com/bluesky-social/atproto/pull/5190