ssh_openssl.h: remove unused declaration

ssh_openssl.h: remove unused declaration

Commit 95993a1df3 ("Refactored SSL initialisation functions")
has declared openssl_set_mydata_index(void) function but
hasn't added definition.

There hasn't been need in this function for 10 years,
so it can be safely removed.

Found by MSVC.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211201092002.162-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23269.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b939fdcfb669bdd57b33e70fd2a2718fcfa626c9)