The Calendar, Lightning is no longer an addon, it is a fully integrated part of Thunderbird. Hence why it cannot be disabled. The Today Pane itself is only shown if you click on the 'Today Pane' in bottom right corner. Otherwise it is just a menu item on the Status Bar. You could disable the 'Status Bar' (View > Toolbars > Status Bar) and hide everything, but it is useful to have that showing for many valid reasons. I do not use Calendar a huge amount but it's Tasks is really useful for setting up reminders.