Update OnScreenStick.cs - fix memory leak

[LukeUnityDev]

Description

Fix memory leak - make sure to unsubscribe started, and canceled actions for m_PointerDownAction on destroy when m_UseIsolatedInputActions is set to true.

Changes made

Added private method OnDestroy.

Testing

Fore testing is required to add OnScreenStick component and check UseIsolatedInputActions, and setup for-example Hold interaction there.

Risk

There is no risk.

[unity-cla-assistant]

All committers have signed the CLA.

[ghost]

@ekcoh thank you for review, I have added

[ekcoh]

Seems problematic to land from fork. I rerouted your fix via https://github.com/Unity-Technologies/InputSystem/pull/2006 and marked your contrib in the CHANGELOG so will close this PR and let the other land when reviewed and approved by the team. Thanks @LukeUnityDev