Clean up examples creation

Create just one single file qt5_examples.7z instead of having them
split into examples_essentials.7z and examples_addons.7z

Also remove examples_essentials.zip and examples_addons.zip as they
are never used.

Change-Id: I853b8efc8c828e9aadc4bb3364e062c58eb3a061
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>