CI: Drop obsolete tags to force jobs to run on whitewhale

This made sense some time ago. Nowdays, the whitewhale runner seems to
not really be reliable, so tagging jobs with it doesn't gain much.

Relates #45

.gitlab-ci.yml

@@ -45,8 +45,6 @@ tests:debian:
 tests:debian_with_bubblewrap:
   image: $CONTAINER_REGISTRY:debian
   stage: test
-  tags:
-    - whitewhale
   script:
   - python3-coverage run --branch -m unittest discover -s tests/
   - python3-coverage report --fail-under=100 -m --include 'libmat2/*'
@@ -54,15 +52,11 @@ tests:debian_with_bubblewrap:
 tests:fedora:
   image: $CONTAINER_REGISTRY:fedora
   stage: test
-  tags:
-    - whitewhale
   script:
   - python3 setup.py test
 
 tests:archlinux:
   image: $CONTAINER_REGISTRY:archlinux
   stage: test
-  tags:
-    - whitewhale
   script:
   - python3 setup.py test