[flexiglass] Remove ScrollViewFields#headsupHeightConsumer

This consumer is never set, because we have choosen another delivery
method for the height from the NSSL to the PlaceHolder, the
NotificationHeadsUpHeight layout modifier.

Bug: 296118689
Test: check if HUNs are displayed in flexiglass
Flag: com.android.systemui.scene_container
Change-Id: I2dc80993b81cfe384b234efd0760eecb289b1e00