Remove fullscreenProgress that is not being used

This flow was previously collected by TTVM and later removed by ag/30431716.

Bug: 390581380
Doc: go/launcher-overview-unified-taskviewmodel
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: OverviewImageTest
Change-Id: Ie0add87740b00da7d1c5e61c048dace543c47a66