Remove unused Sandbox Mode in the gesture navigation tutorial

Sandbox Mode for the gesture navigation tutorial is unused in both the
current and redesigned versions of the tutorial, and can therefore be
removed.

Flag: N/A
Fix: 280641779
Test: Manually went through both versions of the tutorial on handheld,
foldable, and tablet devices

Change-Id: I2f6db47df0fa39458d17221f24827eb43e2cae31