remove var from varToUninstParent when uninstantiating the var
remove var from varToUninstParent when uninstantiating the var

CC @lileicc

With forgetting the past, the size of the partial world inside a particle
should be O(1). But because of varToUninstParent was not cleaned up, the
size of the partial world was O(t). This is why later timesteps ran
slower than earlier timesteps.

time time blog -n 1000 -e blog.engine.ParticleFilter -q 1 ar1.blog >/dev/null

before:
real	1m41.407s
user	2m32.540s
sys	0m0.964s

after:
real	0m26.656s
user	0m30.220s
sys	0m0.536s

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