chore(magnification): cleanup flag usage of...

chore(magnification): cleanup flag usage of use_window_original_touchable_region_when_magnification_recompute_bounds

The bug-fix flag is already soaking in trunkfood full stage for a while.
Since there is no coming issues related to the fix, we can apply the fix
and remove the flag usage.

Bug: 341202777
Flag: NONE
Test: manually
      atest AccessibilityMagnificationTest
Change-Id: I6680fbb3444db65c100638b07b42f39ba9e43f3c