chore(magnification): remove flag delay_show_magnification_button usage

The flag has been already in nextfood about 2 monthes. Remove the flag
usage then we'll delete the flag after released.

Bug: 358504461
Flag: com.android.systemui.delay_show_magnification_button
Test: manually
      atest IMagnificationConnectionTest
      atest MagnificationSettingsControllerTest
      atest WindowMagnificationSettingsTest
Change-Id: I30954a9873ddaf9fc6ac2b1ac3f2fc159b8b5044