refactor: Removes the deprecated ENABLE_EXPLORE_DRAG_AND_DROP feature flag

[michael-s-molina]

SUMMARY

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

The previous value of the feature flag was True and now the feature is permanently enabled.

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 (b2ad74f) 69.00% compared to head (fe3f6b9) 59.08%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26343      +/-   ##
==========================================
- Coverage   69.00%   59.08%   -9.92%     
==========================================
  Files        1948     1948              
  Lines       76062    76034      -28     
  Branches     8492     8478      -14     
==========================================
- Hits        52483    44924    -7559     
- Misses      21399    28943    +7544     
+ Partials     2180     2167      -13     

Flag	Coverage Δ
hive	53.68% <ø> (ø)
javascript	56.52% <100.00%> (+0.02%)	⬆️
presto	53.63% <ø> (ø)
python	61.85% <ø> (-20.68%)	⬇️
sqlite	?
unit	55.89% <ø> (ø)

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