external/conscrypt: remove DHKey.

BoringSSL has DH, but it's not wired up to EVP any longer. It would be
possible to get DHKey working directly using DH because it doesn't use
anything non-standard, but it's probably not worth worrying about.

Bug: 20518803
Bug: 20522271
Change-Id: I7167ba5ae96b0ba914c5759b6293236e4a3302da
3 files changed