Removing rollback logic (last_rollback)

[filipefurtad0]

What we should change and why (this is tech debt)

Two BugsnagJS errors were triggered:

i) when creating a new taxon/child (not able to reproduce)

https://app.bugsnag.com/yaycode/open-food-network-js-uk/errors/5f5f9c8d35c98900171659b8?event_id=5f5f9c8d005dfcffa4760000&i=sk&m=nw

ii) when attempting to delete a taxon/child which is is attributed to a product (reproducible).

https://app.bugsnag.com/yaycode/open-food-network-js-uk/errors/5f5fa317f26a69001738bcf0?event_id=5f5fa317005e00fe89a20000&i=sk&m=nw

Both these are:
ReferenceError: last_rollback is not defined.

Context

This came up while testing #5868. Rollback logic has been flagged for removal - see here.

Impact and timeline

Not sure.