Remove config for LGTM service

The lgtm.com service no longer exists and it's now integrated into
GitHub code scanning[1]. This also means that the config file for it is
obsolete and we can simply remove it.

[1]: https://github.blog/2022-08-15-the-next-step-for-lgtm-com-github-code-scanning/

Signed-off-by: aszlig <[email protected]>