What is the proper way to remove a repeatable group instance from its context?
Assuming the used version is 2017.07u2 (resp. core version 21.3.12) the proper mechanism to remove group instances from your document instance is to use the Editor API.
In general it is highly recommended to used the Editor API for every interaction with a document instance in order to guarantee correct behavior in terms of “side-effects” (e.g. computations and dependent elements).
However, this is changed with the introduction of the BAP Form-Engine, because document instances do not exists any longer. Documents are only plain JSON with some minor exceptions (e.g. dates). Here you can just delete the instance directly.
Thank you 