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 <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Jiawen Geng <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Zijian Liu <[email protected]>