Remove deprecated XR_METAX1_hand_tracking_body_fusion extension

Summary:
Remove the deprecated XR_METAX1_hand_tracking_body_fusion extension
across mobile runtime, PC runtime, OVRPlugin, Unity, and tests.

This Tier 5 (partner experimental) extension is superseded by
XR_META_hand_tracking_wide_motion_mode (Tier 3 public) and
XR_METAX1_hand_tracking_wide_motion_mode2. The runtime already
prefers WMM when both are present, and OVRPlugin.cs explicitly
notes "To be removed when wide motion mode fully ships."

Changes:
- Remove EnabledExtensions enum entry and string mapping
- Remove extension registration from tier list
- Remove body_fusion handling from xrCreateHandTrackerEXT (mobile + PC)
- Remove bodyFusionEnabled telemetry parameter chain
- Remove filterOnBodyEnabled per-frame logger filter
- Remove SetBodyFusionHandPoses/IsBodyFusionHandPosesEnabled from OVRPlugin
- Remove bodyFusionHandPosesEnabled from OVRManager
- Remove body fusion test cases and mocks
- Remove serialized properties from ~54 Unity scene/prefab files
- Delete OVRHandTrackingBodyFusionSample.cs and HandTrackingBodyFusion.unity

Reviewed By: MatthewLangille

Differential Revision: D95136735

fbshipit-source-id: cdffa383f120f5534c7ef566418f565d769ed2b9

Author: Robert Memmott

Assets/StarterSamples/Core/Locomotion/PlayerController.prefab

@@ -360,8 +360,6 @@ MonoBehaviour:
   SimultaneousHandsAndControllersEnabled: 0
   _readOnlyWideMotionModeHandPosesEnabled: 0
   wideMotionModeHandPosesEnabled: 0
-  _readOnlyBodyFusionHandPosesEnabled: 0
-  bodyFusionHandPosesEnabled: 0
   dynamicResolutionVersion: 0
 --- !u!1 &4926590347846174712
 GameObject:

Assets/StarterSamples/Usage/Passthrough.unity

@@ -675,8 +675,6 @@ MonoBehaviour:
   SimultaneousHandsAndControllersEnabled: 0
   _readOnlyWideMotionModeHandPosesEnabled: 0
   wideMotionModeHandPosesEnabled: 0
-  _readOnlyBodyFusionHandPosesEnabled: 0
-  bodyFusionHandPosesEnabled: 0
   dynamicResolutionVersion: 1
 --- !u!114 &236499497
 MonoBehaviour:

Assets/StarterSamples/Usage/Passthrough/Scenes/EnableDisablePassthrough.unity

@@ -1660,8 +1660,6 @@ MonoBehaviour:
   SimultaneousHandsAndControllersEnabled: 0
   _readOnlyWideMotionModeHandPosesEnabled: 0
   wideMotionModeHandPosesEnabled: 0
-  _readOnlyBodyFusionHandPosesEnabled: 0
-  bodyFusionHandPosesEnabled: 0
   dynamicResolutionVersion: 1
 --- !u!114 &234119192
 MonoBehaviour:

Assets/StarterSamples/Usage/Passthrough/Scenes/PassthroughAtStartup.unity

@@ -3280,8 +3280,6 @@ MonoBehaviour:
   SimultaneousHandsAndControllersEnabled: 0
   _readOnlyWideMotionModeHandPosesEnabled: 0
   wideMotionModeHandPosesEnabled: 0
-  _readOnlyBodyFusionHandPosesEnabled: 0
-  bodyFusionHandPosesEnabled: 0
   dynamicResolutionVersion: 0
 --- !u!114 &1906750185
 MonoBehaviour: