[flexiglass] Deletes SceneContainerStartable from main.

Due to a Gerrit user error, we ended up in a situation where only the
main branch has a SceneContainerStartable while both the main and
udc-qpr-dev branches have the SystemUiDefaultSceneContainerStartable.

This is causing build errors like:
https://android-build.corp.google.com/artifact/pending/P60200317/aosp_cheetah-trunk_staging-userdebug/latest/view/logs%2Fbuild_error.log

Bug: 290984671
Test: Gerrit CI
Change-Id: Ia1c5400466ded9584d0f0a4cb39ab7cb2a19ae10