Remove checking of SNI headers

This is only useful when we used the [] proxy because the GFE requires SNI during handshake in order to request the client certificate. The GCP proxy does not need this (it always requests the client certificate). We do not need to check for its existence.

Also removed the checking of internal headers for ssl cert hash used only by the [] proxy.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213059027

docs/flows.md

@@ -1174,7 +1174,6 @@ An EPP flow for login.
     *   Registrar certificate does not match stored certificate.
     *   Registrar IP address is not in stored whitelist.
     *   Registrar certificate not present.
-    *   SNI header is required.
     *   Registrar password is incorrect.
     *   Registrar with this client ID could not be found.
 *   2201