[2/n] Remove use of `persist.wm.debug.desktop_mode_2`

`persist.wm.debug.desktop_mode_2` is no longer needed as desktop
windowing now uses a runtime flag which can be modified using the command: `adb shell device_config put lse_desktop_experience
com.android.window.flags.enable_desktop_windowing_mode (true/false)`.

Flag: NONE
Fixes: 326060855
Bug: 304778354
Test: build & `adb shell device_config put lse_desktop_experience
com.android.window.flags.enable_desktop_windowing_mode true && adb
reboot` desktop windowing mode should be enabled.

Change-Id: I25d509c0912ab3266a32ad992333460332b8b0a2