hgweb: drop support for "manifest" parameter AFAICT (but note that this is the first time I look at hgweb code), the "mercurial" query parameter was removed from rendered pages in 36fa5db79dd5 (hgweb: convert gitweb to NWI, 2006-10-05). Search for "manifest=" in that diff to see why I think it was removed. It's about time we stop looking for the parameter in requests. Differential Revision: https://phab.mercurial-scm.org/D3073