This is just kind of a silly thing to do. NSS doesn't allow them either. Fatal
alerts would kill the connection regardless and warning alerts are useless. We
previously stopped accepting fragmented alerts but still allowed them doubled
up.

This is in preparation for pulling the shared alert processing code between TLS
and DTLS out of read_bytes into some common place.

Change-Id: Idbef04e39ad135f9601f5686d41f54531981e0cf
Reviewed-on: https://boringssl-review.googlesource.com/7451
Reviewed-by: Emily Stark (Dunn) <estark@google.com>
Reviewed-by: David Benjamin <davidben@google.com>