Remove SSL_copy_session_id.

This is the only codepath that allowed a cert_st to be shared between two
ssl_st's. Given that the cert_st currently contains some per-connection and
even per-handshake state, this probably doesn't work.

Remove the function altogether and don't ref-count cert_st.

Change-Id: I66d5346117cb59b6063e7b9b893d1c4b40cb6867
Reviewed-on: https://boringssl-review.googlesource.com/1110
Reviewed-by: Adam Langley <agl@google.com>