Remove unused Flutter apps

Remove unused Flutter apps

Remove unused GSoC prototype Flutter apps flutter-mediaplayer and
flutter-nav to avoid confusion and reduce maintenance burden.

Bug-AGL: SPEC-5216

Change-Id: I0a23a471da22b98ade3deda4006e4626630c4bda
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30151
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account