flatpak-bisect: Remove unused variable

This fixes a warning pointed out by lgtm.com. I checked that the following
still works:
$ PAGER=more flatpak-bisect -b stable org.gnome.Builder log

Closes: #2894
Approved by: matthiasclasen
This commit is contained in:
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f