Remove GitLab CI configuration

With the addition of the GitHub Actions workflow we have found a better
working replacement for the GitLab CI. To that end, we no longer want to
use the latter.
This change removes the corresponding configuration from the repository.

Author: d-e-s-o

CHANGELOG.md

@@ -8,6 +8,7 @@ Unreleased
   extensions
 - Allowed entering of `base32` encoded strings containing spaces
 - Fixed pinentry dialog highlighting some messages incorrectly as errors
+- Switched to using GitHub Actions as the project's CI pipeline
 - Bumped `nitrokey` dependency to `0.9.0`