Not need to show magnification switch UI when magnification scale is changed

In the new design, we show the magnification switch UI when the touch
interaction starts, ends or cancels.
We don't need to notify showing the switch UI when the scale is changed
because the new design includes the case for scale changes.
So we can remove the implementation of onMagnificationScaleChanged.

Bug: 172631612
Test: atest FullScreenMagnificationGestureHandlerTest MagnificationControllerTest
Change-Id: I245b611d8a6a50aeff008257cbb87dfed8fd8962