ci: drop remote-fix workflows; branch is retired

remote-fix is being retired; the v6 codebase it carried will not be
brought forward to v7. Removing the two workflows that only had
meaning while the branch existed:

- auto-sync-upstream.yml — targeted remote-fix; schedule was already
  disabled in 26b7744. With the branch gone, even manual dispatch
  would fail at `Checkout target branch`. The sibling
  auto-sync-upstream-copilot.yml remains as the active template for
  any future per-branch auto-sync setup.

- ghcr-remote-fix.yml — only fired on push to remote-fix. Builds the
  ghcr.io/conversun/cli-proxy-api-plus:remote-fix image. The
  ghcr-feat-copilot.yml remains as the active per-branch GHCR builder.

The :remote-fix image tag on GHCR is left untouched (existing pulls
remain reproducible); delete it manually if reclaiming the namespace
matters.

Author: conversun