[Postfix] Do not remove user agent

data/conf/postfix/anonymize_headers.pcre

@@ -2,7 +2,6 @@ if /^\s*Received:.*Authenticated sender.*\(Postcow\)/
 /^\s*Received:.*Authenticated sender:(.+)/
   REPLACE Received: from localhost (localhost [127.0.0.1]) (Authenticated sender:$1
 endif
-/^\s*User-Agent/        IGNORE
 /^\s*X-Enigmail/        IGNORE
 /^\s*X-Mailer/          IGNORE
 /^\s*X-Originating-IP/  IGNORE