Cealnup released enable_magnification_follows_mouse_bugfix

This is already released.
We also plan to have a new implementation, so before starting a new
work, let's clean this up.

Bug: 354696546
Test: FullScreenMagnificationGestureHandlerTest
Test: MagnificationGestureHandlerTest
Flag: EXEMPT removing com.android.server.accessibility.enable_magnification_follows_mouse_bugfix
Change-Id: I40ddbde78c1a8c1b15a8a29ee3d15784c116dbd8