Remove redundant declarations in ssl_locl.h

Remove the function prototypes for ssl_cert_get0_next_certificate, ssl_set_default_md, tls1_shared_list,
dtls1_send_newsession_ticket, tls1_ctrl, and tls1_callback_ctrl, all of which are not defined.

It also changed the signature of the function pqueue_next to `pitem *pqueue_next(piterator *item)` in
pqueue.c, making it match the prototype in ssl_locl.h. (`piterator *` is equivalent to `pitem **`.)

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4049)