Remove unused environment variable

[bgedik]

Tracking issue

Part of RFC https://github.com/flyteorg/flyte/blob/b3add894fd879d6b6c11e37ba292ae04bf60afeb/rfc/system/5598-deterministic-errors-distributed-training.md#flytepropeller-backend

Why are the changes needed?

With the PR #5795 there is not a need for a generic POD name environment variable. Instead the worker name environment variable is now used.

What changes were proposed in this pull request?

Remove an internal environment variable that was introduced via #5616

How was this patch tested?

Unit tests

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

• Add multi file error aggregation strategy #5795
• Add FLYTE_INTERNAL_POD_NAME environment variable that holds the pod name #5616

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.94%. Comparing base (fa01e76) to head (994f513).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5987      +/-   ##
==========================================
- Coverage   36.94%   36.94%   -0.01%     
==========================================
  Files        1310     1310              
  Lines      131382   131373       -9     
==========================================
- Hits        48540    48531       -9     
  Misses      78629    78629              
  Partials     4213     4213              

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.07% <ø> (ø)
unittests-flytecopilot	22.23% <ø> (ø)
unittests-flytectl	62.39% <ø> (ø)
unittests-flyteidl	6.92% <ø> (ø)
unittests-flyteplugins	53.82% <ø> (-0.03%)	⬇️
unittests-flytepropeller	43.08% <ø> (ø)
unittests-flytestdlib	55.31% <ø> (ø)

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