Assert legacy mode in NSSL goToFullShade

This is no longer needed when scenes are enabled. The only call chain
goes through NPVC, which does not exist with the flag enabled.

Fixes: 348008597
Test: manually verified replacement in ShadeLockscreenInteractorImpl
Flag: com.android.systemui.scene_container
Change-Id: I9e7577afcd262c129e22b32519ba62b8c00fdff7