Remove agl-ivi-demo-flutter-simple image

Remove agl-ivi-demo-flutter-simple image and the recipes for its
flutter-homescreen, flutter-dashboard, and flutter-hvac
applications.  They have been replaced with the new integrated
Flutter homescreen used in the default agl-ivi-demo-flutter image.

Bug-AGL: SPEC-5153

Change-Id: Ib63bf7d4359b43509593226673da06a2fb9ea44c
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29951
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>