viewvcs: cleanup utf8 handling

viewvcs: cleanup utf8 handling

Favor in-place utf8::decode since it's a bit faster without
method dispatch overhead; and don't care about validity just
yet.

HlMod->do_hl itself should return "utf8" strings, since other
parts of our code can use it, so it's not the job of ViewVCS to
post-process HlMod output.