[dynamo] Remove VariableTracker.parents_tracker (#122058)
This is leftover from mutable variable tracker days and no longer needed.
Improves benchmarks/dynamo/microbenchmarks/dynamo_microbenchmarks.py
from 4.2s to 3.9s.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/122058
Approved by: https://github.com/oulgen, https://github.com/anijain2305
ghstack dependencies: #122039, #122043, #122055
