Remove dependency on z3c.unconfigure

[davisagli]

For Plone 6.1 it would be nice to avoid the dependency on z3c.unconfigure, since it isn't a dependency of Plone yet.

This requires:

• remove

  plone.distribution/src/plone/distribution/browser/overrides.zcml

  Line 16 in 9145dd0

  <unconfigure>

  which unconfigures the default admin views (this can happen after Refactor site creation Products.CMFPlone#3981 is merged to make them conditional)
• remove

  plone.distribution/src/plone/distribution/services/overrides.zcml

  Line 16 in 9145dd0

  <unconfigure>

  which helps override the @system endpoint in plone.restapi. Probably the service in plone.restapi should be updated to conditionally add the distribution info if plone.distribution is installed.