.gitlab-ci.yml: remove impure-tests from pipeline

.gitlab-ci.yml

 stages:
 - generate
 - check
-- impure
 
 generate-flake-ci:
   stage: generate
@@ -17,12 +16,3 @@ flake-ci:
       - artifact: flake-ci.yml
         job: generate-flake-ci
     strategy: depend
-
-impure-tests:
-  stage: impure
-  rules:
-  - if: $CI_PIPELINE_SOURCE == "merge_request_event"
-    when: manual
-    allow_failure: true
-  script:
-  - nix run .#run-impure-tests