ci: remove redundant GitLab CI configuration

Removed the entire `.gitlab-ci.yml`, discontinuing the GitLab CI/CD process. This change reflects a shift toward consolidating our CI/CD workflows, potentially to another platform or to streamline our current processes. It's essential to assess the impacts on project deployment and distribution, especially regarding PyPI publishing previously handled by this GitLab CI configuration.

.gitlab-ci.yml

@@ -1,21 +0,0 @@
-image: python:3.10
-
-stages:
-  - test
-  - publish
-
-before_script:
-  - python -V
-  - python -m venv venv
-  - source venv/bin/activate
-  - pip install -U pip
-  - pip install .[all]
-
-publish:
-  stage: publish
-  script:
-    - pip install -U hatchling twine build
-    - python -m build .
-    - python -m twine upload --username __token__ --password ${PYPI_TOKEN} dist/*
-  only:
-    - tags