[Packaging] Remove generate-flat-tree.sh.

Enough time has passed with us using gbp-flat-tree.sh by default in release
and build bots (as well as individually) and nothing has broken. We can
finally retire the old generate-flat-tree.sh script.
Merge pull request #1292 from rakuco/remove-generate-flat-tree

[Packaging] Remove generate-flat-tree.sh.