hgweb: use web.req instead of req.req
We now have access to the modern request type on the
requestcontext instance. Let's access it from there.
While we're here, remove an unused argument from _search().
Differential Revision: https://phab.mercurial-scm.org/D2798
hgweb: stop setting headers on wsgirequest
All commands now go through the new response API. This is dead code.
Differential Revision: https://phab.mercurial-scm.org/D2797