Fixup SSL EX_DATA index

The SSL, SSL_CTX, and SSL_SESSION indices were being referenced
incorrectly in the "_get_ex_new_index" functions.

Remove the STORE EX_DATA index; that functionality is gone.

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>