Remove USE_NEW_ACTIVITY_STARTER flag

This flag is no longer used and can be cleaned up.

Fixes: 278761837
Test: make sure sysui builds.
Change-Id: I2bef5f6f82070900b2a171472428377640e04f4a

packages/SystemUI/src/com/android/systemui/flags/Flags.kt

@@ -714,8 +714,4 @@ object Flags {
     @JvmField
     val SPLIT_SHADE_SUBPIXEL_OPTIMIZATION =
             unreleasedFlag(2805, "split_shade_subpixel_optimization", teamfood = true)
-
-    // TODO(b/278761837): Tracking Bug
-    @JvmField
-    val USE_NEW_ACTIVITY_STARTER = releasedFlag(2801, name = "use_new_activity_starter")
 }