[flexiglass] Removes legacy visibility toggle.

Now that the UI of Flexiglass is very similar to the legacy UI, having
this no longer has value so removing it.

Fix: 291321285
Test: manually verified no crashes and moved through all scenes on a
phone
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I89f1d76d8d4cb985ea12f56079a6b1abed7d0c8b