Avoiding superflous references to MD5 makes it easier to audit the code
to find unsafe uses of it. It also avoids subtly encouraging users to
choose MD5 instead of a better alternative.

Change-Id: Ic78eb5dfbf44aac39e4e4eb29050e3337c4445cc
Reviewed-on: https://boringssl-review.googlesource.com/3926
Reviewed-by: Adam Langley <agl@google.com>