[Dual Shade] Remove references to Dual Shade flag in Notifications.kt.

This is needed to transition from flag-based activation of dual shade to
a setting.

Bug: 388793191
Test: Unit tests still pass.
Flag: com.android.systemui.dual_shade
Change-Id: I446e4b1469d1b0636e4e30e27d083636a5f9f488