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

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.27%. Comparing base (d32709d) to head (e54d908).
⚠️ Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
petastorm/etl/safe_pickle.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ selitvin
❌ Yevgeni Litvin

---

Yevgeni Litvin seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}