Remove SSL_CTX_select_current_cert.

Without certificate slots this function doesn't do anything. It's new in
1.02 and thus unused, so get rid of it rather than maintain a
compatibility stub.

BUG=486295

Change-Id: I798fce7e4307724756ad4e14046f1abac74f53ed
Reviewed-on: https://boringssl-review.googlesource.com/5352
Reviewed-by: Adam Langley <agl@google.com>