apertis-pkg-merge-updates: Do not abort failed merges

When a merge fails, it's nice to let developers use the same tools as used by the CI to reproduce and fix the issue.

Unfortunately, apertis-pkg-merge-updates cleans up after itself a bit too much, aborting the merge and thus preventing developer from fixing it. Stop doing that so that developers can handle the conflicts, GitLab CI will clean up after itself in any case.