Remove legacy callbacks between recents and split screen

Removes legacy recents callbacks between recents and split screen since
there's no need to interact with RecentsActivityStartingEvent and
RecentGrowingEvent after deprecated legacy recents implmentaton.

Bug: 161116823
Test: atest SystemUITests
Test: manual check recents behavior while in split screen mode
Change-Id: Iee1546d41ae2b1a8e4162cbb2fa599162a5b308f