Remove HelloRequest special-case in DTLS message parsing.

We don't support DTLS renego. Removing this separately from the rewrite to call
out intentionally dropping this logic.

Change-Id: Ie4428eea0d2dbbb8b4b8b6474df4821de62558cc
Reviewed-on: https://boringssl-review.googlesource.com/3761
Reviewed-by: Adam Langley <agl@google.com>