ssl-opt.sh now takes care of looking at the files in the
opt-testcases subdirectory, so use ssl-opt.sh directly
in walk_ssl_opt_sh().
* Revert commit f17a60f147ec3ba6a9f76da38d3802a7f9f05227.
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>