delete folder vendor

[pfi79]

#5037

I think that the vendor folder is already unnecessary and can be deleted.
It is more of a hindrance.

[denyeart]

Whether to include vendor directory or not was discussed early in the project. It was decided to check in the vendor folder to ensure all dependencies remain available for every version of Fabric. Basically it is the first Pro here:

https://stackoverflow.com/questions/47346607/is-it-best-practice-to-commit-the-vendor-directory

It's the only way to get truly reproducible builds, as it guards against upstream renames, deletes and commit history overwrites.