[HDRP] Change the behavior of custom passes when the volume is disabled

[alelievr]

Purpose of this PR

Fix this case: https://fogbugz.unity3d.com/f/cases/1300022/

To fix the domain reload issue, I changed the behavior of the custom pass volume:
Before the fix, disabling a custom pass volume, did not really impact the passes in the volume, they were just not executed.
Now with this fix, the function Cleanup() is called when the Custom Pass Volume component is disabled, which allows releasing the resources allocated in the passes.

Note that it shouldn't change the execution behavior of the passes.

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Testing status

Tested that the Setup and Cleanup function of the custom passes are executed correctly when:

• domain reload happens
• enter play mode
• disable gameobject
• disable custom pass volume component

[pmavridis]

Looks good. I have double-checked that OnDisable is called every time we trigger a destroy, so the change makes sense.