refactor: Removes the deprecated DISABLE_DATASET_SOURCE_EDIT feature flag

[michael-s-molina]

SUMMARY

As part of the 4.0 approved initiatives, this PR removes the DISABLE_DATASET_SOURCE_EDIT feature flag.

The previous value of the feature flag was False and now the feature is permanently removed.

TESTING INSTRUCTIONS

CI should be sufficient for merging this PR. We'll do a complete testing of 4.0 after all approved proposals are merged.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5e85f5c) 69.18% compared to head (d84b443) 69.15%.
Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26331      +/-   ##
==========================================
- Coverage   69.18%   69.15%   -0.03%     
==========================================
  Files        1945     1947       +2     
  Lines       75971    76008      +37     
  Branches     8467     8480      +13     
==========================================
+ Hits        52559    52567       +8     
- Misses      21225    21260      +35     
+ Partials     2187     2181       -6     

Flag	Coverage Δ
hive	53.68% <ø> (+<0.01%)	⬆️
javascript	56.49% <100.00%> (-0.03%)	⬇️
mysql	78.07% <ø> (-0.01%)	⬇️
postgres	78.17% <ø> (-0.01%)	⬇️
presto	53.63% <ø> (+<0.01%)	⬆️
python	82.86% <ø> (-0.01%)	⬇️
sqlite	76.83% <ø> (-0.01%)	⬇️
unit	55.80% <ø> (+0.01%)	⬆️

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