Remove duplicate CIE stub; PR #71 owns the canonical custom-inference-endpoint.mdx

The CIE page is owned by PR #71 (hyc/plan-updates-byok-cie), which also
adds the matching src/sidebar.ts entry. Removing the stub here so the
PR #72 \u2192 umbrella merge auto-resolves cleanly after #71 lands.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: hongyi-chen