Remove ssl_cert_inst()

It created the cert structure in SSL_CTX or SSL if it was NULL, but they can
never be NULL as the comments already said.

(Imported from upstream's 2c3823491d8812560922a58677e3ad2db4b2ec8d.)

Change-Id: I97c7bb306d6f3c18597850db9f08023b2ef74839
Reviewed-on: https://boringssl-review.googlesource.com/4042
Reviewed-by: Adam Langley <agl@google.com>