Remove docked task mode divider shadow

After the docked task mode was removed in ag/20738979, its divider
shadow was left mistakenly enabled. Remove it entirely since it is
no longer ever used.

Bug: 264246748
Test: Open Settings, no line shadow at the bottom is visible
Change-Id: Ib8cf1d352ae077acbd257cb3bfde79bea0fd6945