Remove per_user_display_window_settings flag in WM server.

The per_user_display_window_settings flag is in Nextfood for 4+ weeks,
we no longer need this bug fix flag.
No CTS tests reference this flag, confirming safe removal.

Bug: 346668297
Test: atest WmTests:DisplayWindowSettingsProviderTests
Flag: EXEMPT removing com.android.window.flags.per_user_display_window_settings
Change-Id: Ic43cb3159bb85eefabadf66e8759e73293cb3d90