Remove remnants of DH key support.

7a1929018f423157f9e85dbf368e0b30561c5915 removed most of it, but remnants in
NativeCrypto remained with no callers. Although the APIs are still present in
BoringSSL, they always fail.

Change-Id: Iaff8a142774095e24f08a512956e05272b681b70
2 files changed