CI: Remove job "build-valgrind-memory-check"
Everything that this job does is covered by the more extensive
integration test job integration-test-valgrind-memory-check
.
There also is a potential race condition which makes this test flaky,
see #157 (closed).
We will keep #157 (closed) open and I will try to track this down ASAP, but this simple change should for now resolve our issues with the CI and could speed up our pipelines without a loss in coverage.