tools: remove unused lint-pr-commit-message.sh

`tools/lint-pr-commit-message.sh` was previously used to lint the
commit message of the first commit in a pull request but is now no
longer used -- commit message linting is now done in a GitHub actions
workflow which does not call this script.

PR-URL: #39120
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>