remove code for undocumented setting `instance_id`

[lindsay-stevens]

There is a setting instance_id which accepts any string. When provided, the value is used in the instanceID binding attribute e.g. jr:preload="value", instead of the default jr:preload="uid". However, there are limited preload attributes available in the spec, and only uid does not require an extra jr:preloadParams. It's not clear what happens if some other arbitrary string is used.

It seems that this setting added in Feb 2013 as instanceID then renamed to instance_id about a week later. The default value was a calculate using concat('uuid:', uuid()), until it was changed to a preload with uid for #94. The aim of #94 was to get a stable identifier that worked with drafts and from that ticket and related forum discussion there doesn't seem to be any reason to use something other than the uid preload.

I could not find any current usages or mentions of this parameter outside of pyxform (or it's forks/clones). It's a very common string though so it's possible I missed something in the noise.