chore: Removes deprecated feature flags for 3.0

[michael-s-molina]

SUMMARY

Removes deprecated feature flags for 3.0:

• ALLOW_DASHBOARD_DOMAIN_SHARDING
• DISPLAY_MARKDOWN_HTML
• FORCE_DATABASE_CONNECTIONS_SSL

This is a task of the Superset 3.0 initiative.

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 #23663 (3370739) into master (b39edc1) will increase coverage by 1.78%.
The diff coverage is 0.00%.

❗ Current head 3370739 differs from pull request most recent head af2d641. Consider uploading reports for the commit af2d641 to get more accurate results

@@            Coverage Diff             @@
##           master   #23663      +/-   ##
==========================================
+ Coverage   66.30%   68.08%   +1.78%     
==========================================
  Files        1922     1922              
  Lines       74048    74046       -2     
  Branches     8104     8104              
==========================================
+ Hits        49094    50416    +1322     
+ Misses      22882    21556    -1326     
- Partials     2072     2074       +2     

Flag	Coverage Δ
hive	53.18% <ø> (?)
javascript	54.13% <0.00%> (+<0.01%)	⬆️
mysql	79.21% <ø> (ø)
postgres	79.28% <ø> (-0.02%)	⬇️
presto	53.09% <ø> (?)
python	83.13% <ø> (+3.71%)	⬆️
sqlite	77.79% <ø> (ø)
unit	53.02% <ø> (?)

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

Impacted Files	Coverage Δ
...s/superset-ui-core/src/components/SafeMarkdown.tsx	85.71% <0.00%> (+19.04%)	⬆️
...ackages/superset-ui-core/src/utils/featureFlags.ts	100.00% <ø> (ø)
...end/src/features/databases/DatabaseModal/index.tsx	45.36% <ø> (-0.12%)	⬇️
superset-frontend/src/utils/hostNamesConfig.js	73.33% <ø> (ø)
superset/config.py	91.86% <ø> (ø)

... and 95 files with indirect coverage changes

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