Cleanup: Remove setHasNavigationBar()

The functionality of setHasNavigationBar(true) has been merged into
onDisplayAddSystemDecorations(), and setHasNavigationBar(false) has been
merged into onDisplayRemoveSystemDecorations().

Bug: 390591772
Flag: EXEMPT refactor
Test: adb shell settings put secure mirror_built_in_display {1|0}
Change-Id: I2e4cabc8277ebeadf53a3f49bc96cec0ab5b550f