refactor: remove X-Forwarded-Prefix from Vary header in SSR redirect utility

[alan-agius4]

This is no longer needed since now X-Forwarded-Prefix is validated by the users.

[gemini-code-assist]

Code Review

This pull request removes the logic that automatically adds the X-Forwarded-Prefix header to the Vary response header during redirects. However, this change introduces a high-severity cache poisoning risk; since the redirect Location depends on the X-Forwarded-Prefix value, omitting it from the Vary header can cause CDNs or reverse proxies to serve incorrect cached redirects to users on different proxy paths.

[gemini-code-assist]

Removing the logic that automatically adds X-Forwarded-Prefix to the Vary header can lead to cache poisoning. Even if the header is validated, the redirect Location still depends on its value (as seen in AngularAppEngine.redirectBasedOnAcceptLanguage in app-engine.ts).

When a redirect response is cached by a CDN or reverse proxy, it must vary by all headers used to generate the Location URL. If X-Forwarded-Prefix is omitted from Vary, a user accessing the site through a different proxy prefix might receive a cached redirect to the wrong path. If the intention is to make this utility more generic, the responsibility for adding X-Forwarded-Prefix to Vary should be moved to the caller, but currently, AngularAppEngine does not appear to handle this, which introduces a regression in cache safety.