CI: Remove integration of retired lgtm.com

Company was aquired by Github in 2019. Service was switched off in Dec 2022,
replaced by GitHub code scanning. Fixes #1260

See
- https://github.blog/news-insights/product-news/the-next-step-for-lgtm-com-github-code-scanning/

.lgtm.yml

@@ -1,10 +0,0 @@
-path_classifiers:
-  library:
-    - "mitogen/compat"
-    - "ansible_mitogen/compat"
-queries:
-  # Mitogen 2.4 compatibility trips this query everywhere, so just disable it
-  - exclude: py/unreachable-statement
-  - exclude: py/should-use-with
-  # mitogen.core.b() trips this query everywhere, so just disable it
-  - exclude: py/import-and-import-from

README.md

@@ -7,5 +7,3 @@
 <a href="https://mitogen.networkgenomics.com/">Please see the documentation</a>.
 
 ![](https://i.imgur.com/eBM6LhJ.gif)
-
-[![Total alerts](https://img.shields.io/lgtm/alerts/g/mitogen-hq/mitogen.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/mitogen-hq/mitogen/alerts/)

docs/changelog.rst

@@ -24,6 +24,7 @@ In progress (unreleased)
 * :gh:issue:`1243` :mod:`mitogen`: Pass first stage, context name, & preamble
   size as seperate **argv** arguments
 * :gh:issue:`1218` :mod:`ansible_mitogen`: Remove maximum Ansible version check
+* :gh:issue:`1260` CI: Remove integration of retired lgtm.com
 
 
 v0.3.31 (2025-11-05)