remove code for undocumented setting `omit_instanceID`

[lindsay-stevens]

There is a setting omit_instanceID which accepts yes / no / boolean aliases. When active, the setting results in the instanceID being omitted from the survey-level meta element. However, instanceID is required metadata in the ODK spec.

It seems that this setting was added in March 2012. The change commit switched from always emitting ./orx:meta/orx:instanceID, to emitting ./meta/instanceID unless omit_instanceID is active; the orx namespace is https://openrosa.org/xforms whereas the current default namespace is http://www.w3.org/2002/xforms. It's not clear why this setting was added but maybe it was meant to help systems transition to the new namespace without emitting duplicate elements (in different namespaces). Or alternatively, the commit adds a comment "Warning, this will cause errors on old versions of collect because uuid is not a function" so perhaps it was for Collect compatibility.

I could not find any current usages or mentions of this parameter outside of pyxform (or it's forks/clones).