Remove X509 parameter from ssl_cert_type.

No current use of ssl_cert_type passes a NULL EVP_PKEY, so it can be simplified
a little.

Change-Id: I2052cc3b6069cd30e4685ba8a6d0014016a4d712
Reviewed-on: https://boringssl-review.googlesource.com/2620
Reviewed-by: Adam Langley <agl@google.com>