chore(ci): remove obsolete sync-develop workflow

[lxjmaster]

Related Issue

Refs Gridltd-DevOps/.github#2050
Resolves Gridltd-DevOps/.github#2053

Summary

• Remove obsolete .github/workflows/sync-develop.yml from this repository.
• Owner decision recorded in Gridltd-DevOps/.github: future develop branch synchronization is no longer required for this cleanup scope.
• Scope is intentionally limited to this one workflow file.

Test Plan

• Preflight: .github/workflows/sync-develop.yml existed on the default branch before this PR.
• Diff review: this PR deletes exactly .github/workflows/sync-develop.yml.
• Post-merge: verify GitHub Contents API returns 404 for the workflow on the default branch.

Boundary:

• No repository deletion.
• No branch deletion.
• No application code changes.
• No ruleset, branch protection, required-check, runner, secret, GitHub App, Fly, Redis, deployment, release, workflow dispatch, or broad sync mutation.