Internalizes SCT_verify and removes SCT_verify_v1

SCT_verify is impossible to call through the public API (SCT_CTX_new() is
not part of the public API), so rename it to SCT_CTX_verify and move it
out of the public API.

SCT_verify_v1 is redundant, since SCT_validate does the same verification
(by calling SCT_verify) and more. The API is less confusing with a single
verification function (SCT_validate).

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>