Remove support on both sides for *_fixed_(ec)dh client auth.

In the fixed_ecdh case, it wasn't even implemented, but there was stub code for
it. It complicates the ClientKeyExchange (the client parameters become implicit
in the certificate) and isn't used.

Change-Id: I3627a37042539c90e05e59cd0cb3cd6c56225561
Reviewed-on: https://boringssl-review.googlesource.com/1563
Reviewed-by: Adam Langley <agl@google.com>