Remove desktop tile from proto 1 of desktop windowing

Separate desktop tile in overview that groups together all open desktop
tasks, is only available with prototype 2. In prototype 1 of desktop
windowing, each task will show up as a separate tile.

Bug: 266711786
Test: with proto1 enabled, launch some apps, open overview and observe each
  app having their own tile
Test: with proto1 enabled, tap on any tile in overview and observe that
  task is added to existing apps on desktop
Change-Id: Ic9385ff047e0c8908a74d88f88ae5c46e0309cfd