InputManager: Removed pointerup check that prevented event from being processed.

[PolygonalSun]

During some testing, I found that there was a bug that was causing one of scene.onPointerObservable.notifyObservers calls to not be called if more than one button was pressed. This could be reproduced in Chromium, Firefox, and Safari browsers.

Basic Repro Steps:

1. Open Basic PG
2. Press left button, middle button, then right button (Do not let go of any of them)
3. Release them in this order while moving around a bit (Right, Left, Middle)
4. Now camera movement should be locked to your cursor movement

It should be noted that the DeviceSourceManager/DeviceInputSystem is still storing the correct values. This fix removes the check from the InputManager.

[azure-pipelines]

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