removed screenshot*.png files

removes ~10% unused weight of the package

This removes the screenshots from the code and thereby from the wp.org package.

All assets used on wp.org should be moved to the `assets` branch in the wp.org-SVN repo.
See the "orphan" branch https://github.com/cfoellmann/the-events-calendar/tree/assets - if use this PR you can pull the independent branch to track wp.org assets.