Remove very old pickle compatibility code modifying old atg package names

[selitvin]

This is code cleanup. The code was meant to support opening some old datasets that contained pickled data with pre-petastorm names.

Can not specify selitvin:selitvin/remove_legacy as an upstream, so github shows two commits in this PR. Please review only the second commit as the first is part of #640

[codecov]

Codecov Report

Merging #702 (e54d908) into master (d32709d) will increase coverage by 0.00%.
The diff coverage is 86.66%.

@@           Coverage Diff           @@
##           master     #702   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files          85       85           
  Lines        5084     5070   -14     
  Branches      787      784    -3     
=======================================
- Hits         4386     4374   -12     
+ Misses        559      558    -1     
+ Partials      139      138    -1     

Impacted Files	Coverage Δ
petastorm/etl/safe_pickle.py	81.81% <81.81%> (ø)
petastorm/etl/dataset_metadata.py	87.33% <100.00%> (ø)
petastorm/etl/rowgroup_indexing.py	64.51% <100.00%> (ø)

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✅ selitvin
❌ Yevgeni Litvin

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