fetchGit: Drop git+ from the url attribute

This was already dropped in `inputFromURL()`, but not in
`inputFromAttrs()`. Now it's done in `fixGitURL()`, which is used by
both.

In principle, `git+` shouldn't be used in the `url` attribute, since
we already know that it's a Git URL. But since it currently works, we
don't want to break it.

Fixes #14429.

src/libfetchers/git.cc

@@ -168,8 +168,6 @@ struct GitInputScheme : InputScheme
             return {};
 
         auto url2(url);
-        if (hasPrefix(url2.scheme, "git+"))
-            url2.scheme = std::string(url2.scheme, 4);
         url2.query.clear();
 
         Attrs attrs;