Move deprecated WebVR-specific extensions to WebVR

[ddorwin]

GamepadHand, GamepadPose, pose, and hand in https://w3c.github.io/gamepad/extensions.html were only used for WebVR, which is deprecated in favor of WebXR where this functionality is addressed in a different way.

To avoid confusion, these should be removed from the Gamepad Extensions draft. It still makes sense to document what some browsers have implemented for WebVR, so these could be moved to the WebVR spec, which already has a Gamepad Interface extension section.

That would leave https://w3c.github.io/gamepad/extensions.html as only containing haptics. For clarity, it may make sense to retitle it as Gamepad Haptics or similar.

/cc @toji @nondebug

[ddorwin]

Even the haptics part (https://w3c.github.io/gamepad/extensions.html#dom-gamepad-hapticactuators) appears to be outdated as it does not match #68. Thus, this document is also misleading for non-VR-related readers. I propose deleting it or replacing it with a stub that references the WebVR section and #68.