Removing URL files when HQ jobs are cancelled

[Schlevidon]

After #45 all old URL files are removed when starting the load balancer. Should we still remove the URL file whenever a HQ job is cancelled?

• Not removing the files might be helpful for debugging and keeping them wouldn't clutter the system too much, since they get deleted before each run anyways.
• If we decide to keep the current behavior, the code should be refactored to use the std::filesystem library instead of plain bash.

https://github.com/UM-Bridge/umbridge/blob/b9f3b1ec93a23adfea3bdfca0bdd57cea6fcad0f/hpc/LoadBalancer.hpp#L142

[linusseelinger]

I think we should still delete the url files after use. There might be millions of runs, so at some point the file system may become unhappy. We might think about some kind of verbose/debug output mode though if problems turn out frequent in practice. Good point about std::filesystem!