Remove STLState.seekTo{Scene,ShowOverlay,HideOverlay}

This CL removes the seek methods on MutableSTLState, which were
initially written for the Compose QS but have now been superseded by
custom transitions in ag/29386583.

Bug: 378470603
Test: N/A
Flag: EXEMPT deleting unused code
Change-Id: Iffd5e17688b22fb75e1a1ca3acfbc406ceb84c91