Remove DSA-based cipher suites and client auth.

DSA is not connected up to EVP, so it wouldn't work anyway. We shouldn't
advertise a cipher suite we don't support. Chrome UMA data says virtually no
handshakes end up negotiating one of these.

Change-Id: I874d934432da6318f05782ebd149432c1d1e5275
Reviewed-on: https://boringssl-review.googlesource.com/1566
Reviewed-by: Adam Langley <agl@google.com>