This reverts the following part of aed4f303:

* 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.