Remove TODO about DTLS cookie lengths.

The limit increased from 32 to 255 between DTLS 1.0 and DTLS 1.2.

Change-Id: I329a59f9ba2bccc70282e2b47679c57b67e5ed43
Reviewed-on: https://boringssl-review.googlesource.com/2242
Reviewed-by: Adam Langley <agl@google.com>