chore(feature flag): removing unused ENFORCE_DB_ENCRYPTION_UI flag

[rusackas]

SUMMARY

The ENFORCE_DB_ENCRYPTION_UI doesn't seem to be used anywhere. This should not be considered a breaking change since it appears to be unimplemented in the first place. This PR simply sweeps up.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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

Merging #23640 (5a5b7af) into master (b44d203) will not change coverage.
The diff coverage is n/a.

❗ Current head 5a5b7af differs from pull request most recent head e23823b. Consider uploading reports for the commit e23823b to get more accurate results

@@           Coverage Diff           @@
##           master   #23640   +/-   ##
=======================================
  Coverage   67.71%   67.71%           
=======================================
  Files        1918     1918           
  Lines       74157    74157           
  Branches     8053     8053           
=======================================
  Hits        50219    50219           
  Misses      21885    21885           
  Partials     2053     2053           

Flag	Coverage Δ
hive	52.72% <ø> (ø)
mysql	78.53% <ø> (ø)
postgres	78.60% <ø> (ø)
presto	52.65% <ø> (ø)
python	82.44% <ø> (ø)
sqlite	77.11% <ø> (ø)
unit	52.62% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/config.py	91.86% <ø> (ø)

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