feat(registry): importer: remove blob transfer

Related to: Exploring the possible failure modes for in pla... (#901 - closed).

This MR removes all code and documentation related to blob transfer, both for the importer command and the import API. We're able to do this for the API since blob transfer happened transparently beneath the API contract. The reason for the removal is to reduce the possible failure modes and general complexity for the import command, so that it is easier to investigate possible failure modes that can happen when uses run the import command according to the design from Design an In Place Migration Procedure for Self... (#884 - closed) which does not allow blob transfer.

Removing everything at once should allow for a cleaner revert if we need to bring this feature back.