Fix #12549: Make import summary compulsory.

This PR builds on top of https://github.com/openmicroscopy/openmicroscopy/pull/2950 and removes the `--summary` parameter, making the import summary a non-optional part of the import progress output.

To test:
- check that a combination of single images/MIFs/plates produces proper results (nothing changed in the logic since the previous PR),
- verify that the summary isn't printed when `-f` is user or no import candidates are found,
- verify that http://ci.openmicroscopy.org/view/5.1/job/OMERO-5.1-merge-integration-python/ is green (CLI tests).

/cc @joshmoore