This reverts the following part of 5d9c30e:

* Upon bt5 installation, install portal_type* items before bt5 {Document,
  Extensions...} as corresponding Portal Type are required once they have been
  migrated to ZODB Components.

This obviously does not work as the Document can be used as Type Class and
also because tool_item is required before portal_types for example (see
test_type_provider).

When bootstrapping erp5_core and installing for example an Extension
Component, this will try to load Extension Component Portal Type which does
not exist yet but fallback on its document class instead, which is enough
until Extension Component Portal Type is installed.