ci: drop auto-retarget-main-pr-to-dev workflow on this fork

The workflow retargets every PR with base=main to base=dev. This
serves upstream router-for-me/CLIProxyAPI's main/dev branch model,
but this fork doesn't have a 'dev' branch — every main PR triggers
a failed run with:

  Failed to retarget PR #N to dev: Validation Failed:
  {"message":"Proposed base branch 'dev' was not found", ...}

The retarget itself fails (catch + return), so the PR base is left
intact — but every main PR shows a red 'retarget' check, which is
noise. Removing the file on the default branch disables the
pull_request_target trigger fork-wide.

Author: conversun