commands/migrate: always migrate all packages on every registry
Having to specify if one wants to operate on all packages or on user DB
only confuses both consumers (e.g. boot hook) and the author of the code
(it was removing packages in @all
db only for the first choice).
This commit removes that choice, and simplify the command's option to
just 1: --for-user
. This commit also make sure the old app are
removed from all registries, not just the one the new app resides.
Related: #17 (closed)