Remove schema shell from 3.0

Remove the schema shell. I don't think we are going to have time to
build a proper replacement for 3.0.0. I also think that migrations are
something that would be better handled in a plugin where ideas can be
iterated more quickly and compatibility constraints are less strict.
Merge pull request #2146 from markstory/3.0-no-schema-shell

Remove schema shell from 3.0