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

chore(magnification): cleanup flag usage of hide_magnification_always_on_toggle_when_window_mode_only

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: 341203230
Flag: NONE
Test: manually
      atest MagnificationAlwaysOnPreferenceControllerTest
Change-Id: I7e6ef8e03e7b7291d2faa15351669a91a50f59a5