fix: remove phpstan from gitlab CI pipeline

The `.gitlab-ci.yml` file was updated to remove the `phpstan` job. This change means we will no
longer run static analysis automatically on our CI pipeline. Further discussions may be needed on
whether to replace this with another tool or integrate it differently.

.gitlab-ci.yml

@@ -15,7 +15,3 @@ phpunit-php8.2:
 phpunit-php8.3:
   rules:
     - when: never
-
-phpstan:
-  script:
-    - QUIQQER_OTHER_AUTOLOADERS=KEEP ./tools/phpstan --no-progress --memory-limit=-1 --debug