remove duplicate build prevention logic

First of all docker has caching skills that make duplicate builds almost a non-issue. For example see here: https://0xacab.org/leap/docker/-/jobs/16311

In addition the logic based on the last previous build in pipelines is flaky. What if my last pipeline failed and i fixed the flaw in say .gitlab-ci.yml and want to build now? What if the last pipeline was from a different branch and there seems to be a difference in some of the dockerfiles but exactly the same docker file was build before?

And last but not least... this logic prevents triggering new builds of containers with a simple click on the "Run pipeline" button here: https://0xacab.org/leap/docker/pipelines

So all in all I think we are better off triggering builds all the time since running the exact same build again will be super fast due to caching.