PrimCompositionQuery garbage collection fix

[fabal]

Description of Change(s)

When a PrimCompositionQuery python is created then garbage collected, the underlying c++ objects are not properly destructed. Among other things, this prevents unused layers to be discarded.

from pxr import Sdf, Usd

def func():
    s = Usd.Stage.CreateInMemory()
    Usd.PrimCompositionQuery(s.GetPseudoRoot())

func()

assert not Sdf.Layer.GetLoadedLayers()

Fixes Issue(s)

• Layers are not destroyed upon the garbage collection of a python PrimCompositionQuery object

[jtran56]

Filed as internal issue #USD-6301