removing the check_gn_format.py file from ci/ (#41397)

Delete obsolete empty script.

fixes:  flutter/flutter#100741

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

- [+] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [+] I signed the [CLA].

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: andoriyaprashant