clean_job_logs: Remove backup mechanism

What does this MR do and why?

When ripgreping through all log files, the _backup folder would automatically be included. This seems unnecessary so let's just remove it.

After all, the logs will most likely have been obtained by download_ci_logs.rb, and could simply be re-downloaded.

Screenshots, recordings or output↵

Before	After
$ gls_clean_jobs_log_for_diff_ing CI-*.log	$ gls_clean_jobs_log_for_diff_ing CI-*.log
Clean up further by copy-paste-ing and completing this …	Clean up further (but without backup into '_job_log_backups') by copy-paste-ing and completing this
$ ls	$ ls
_job_log_backups CI-….log …	CI-….log …

How to set up and validate locally

Via the normal USAGE instructions with any example job log.

MR acceptance checklist

• I've tested this change on: macOS/Linux.
• This change needs no other test and thus no review, because these are just string and shellcheck changes. I'll merge it myself.
  • This does need at least 1 additional test on another machine, ideally using a different OS. I've facilitated this via: Slack thread / MR review.