Remove rsa_md5, md5, and sha1 fields from SSL_CTX.

Just use the normal API for them.

Change-Id: Ibb5988611a86e8d39abda1e02087523d98defb51
Reviewed-on: https://boringssl-review.googlesource.com/1555
Reviewed-by: Adam Langley <agl@google.com>