refactor: Removes the deprecated ENABLE_TEMPLATE_REMOVE_FILTERS feature flag

[michael-s-molina]

SUMMARY

As part of the 4.0 approved initiatives, this PR removes the ENABLE_TEMPLATE_REMOVE_FILTERS 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

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f8d2561) 69.07% compared to head (9f2147f) 69.07%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
superset/models/helpers.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26345      +/-   ##
==========================================
- Coverage   69.07%   69.07%   -0.01%     
==========================================
  Files        1930     1930              
  Lines       75300    75299       -1     
  Branches     8429     8429              
==========================================
- Hits        52015    52014       -1     
  Misses      21138    21138              
  Partials     2147     2147              

Flag	Coverage Δ
hive	53.58% <50.00%> (-0.01%)	⬇️
javascript	56.48% <ø> (ø)
mysql	77.87% <50.00%> (-0.01%)	⬇️
postgres	77.97% <50.00%> (-0.01%)	⬇️
presto	53.53% <50.00%> (-0.01%)	⬇️
python	82.77% <50.00%> (-0.01%)	⬇️
sqlite	77.55% <50.00%> (-0.01%)	⬇️
unit	56.02% <0.00%> (+<0.01%)	⬆️

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