To keep the upgrade as simple as possible, we just overwrite the existing installation.
This means that the vendor folder will contain files used by all the previous versions of webtrees that you installed, as well as the current version.
It is already a challenge to make the upgrade script run within the time/memory limits of many servers.
It is also possible that people may have modified their own installation and added additional files.
By default, webtrees only writes to the folder '/data/' and your media folders (which defaults to '/data/media').
So, to remove *all* outdated files, you could (a) delete everything else and then (b) manually upload the files from 1.7.12
> For example, can I delete tecnickrom, tecnick, symfony, and patchwork?
Well, two of these are required and two can be deleted.
You could compare your /vendor/ folder to the one that came with your version of webtrees:
github.com/fisharebest/webtrees/tree/1.7.11/vendor