Remove cookie_len setting in dtls1_new.

This should have been removed with its dtls1_clear cousin in
8c88153465ed4eb657ffc8dedd9d37f72ad3d87d.

Change-Id: Ibf4ee67348f603285b26766568cbb92183b62cee
Reviewed-on: https://boringssl-review.googlesource.com/2823
Reviewed-by: Adam Langley <agl@google.com>