Consider removing `vendor` function if it can't be made useful

There isn't a small number of fixed values that `target_vendor` can have, the strings that `target::vendor` returns are just a few that I happened to see in the docs. I can't think of a way to get the `vendor` string, so I'm considering removing the function, since it's not particularly useful, and can't be made complete.

Start by reviewing the `vendor` function and the `target_vendor` values mentioned in the issue, then compare them with the documented strings returned by `target::vendor`. The issue does not define whether removal or a more complete implementation is desired, so a maintainer decision is needed before the work has a clear completion condition.

Written by the indexing model from the issue text.