Remove unreached header forwarding code in AphrontRequest
Summary:
Static code analysis complained `Cannot unset offset 'Authorization'|'Host' on array<int<0, max>, array{string, mixed}>.`
Per avivey's look at the code, we only ever get headers in the form of X-Hgargs-* in $headers anyway, so the special-case here won't ever happen. Delete this code.
Also see similar code at the bottom of `AphrontHTTPProxyResponse.php`.
Test Plan: Read the code.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25743