Remove duplicate setting of SSL_OP_SINGLE_DH_USE option.

Remove duplicate setting of SSL_OP_SINGLE_DH_USE option.

Commit c0a15e07c moved the setting of OpenSSL's SSL_OP_SINGLE_DH_USE option
into a new subroutine initialize_dh(), but forgot to remove it from where
it was.  SSL_CTX_set_options() is a trivial function, amounting indeed to
just "ctx->options |= op", hence there's no reason to contort the code or
break separation of concerns to avoid calling it twice.  So separating the
DH setup from disabling of old protocol versions is a good change, but we
need to finish the job.

Noted while poking into the question of SSL session tickets.