docs(conf) remove old option that was incorrectly ressurected
docs(conf) remove old option that was incorrectly ressurected

The `ssl` option no longer exists. It was added back to
`kong.conf.default` in 4fa780a2, but it should be removed
from both `kong.conf.default` and the website documentation.

From #4250
Kong/kong,Kong/kong,Kong/kong,Mashape/kong