chore: remove deprecated apis on superset, testconn, validate_sql_json, schemas_access_for_file_upload

[dpgaspar]

SUMMARY

Removes old deprecated APIs:

• /superset/testconn
• /superset/validate_sql_json/
• /superset/schemas_access_for_file_upload
• /superset/extra_table_metadata/

Effort on removing deprecated APIs from /superset: #24332

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 #24354 (500e824) into master (c04bd4c) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 500e824 differs from pull request most recent head 6f5efe3. Consider uploading reports for the commit 6f5efe3 to get more accurate results

@@            Coverage Diff             @@
##           master   #24354      +/-   ##
==========================================
- Coverage   69.05%   69.02%   -0.04%     
==========================================
  Files        1903     1903              
  Lines       74378    74275     -103     
  Branches     8110     8110              
==========================================
- Hits        51363    51268      -95     
+ Misses      20903    20895       -8     
  Partials     2112     2112              

Flag	Coverage Δ
hive	53.98% <100.00%> (+0.09%)	⬆️
mysql	79.34% <100.00%> (-0.04%)	⬇️
postgres	79.41% <100.00%> (-0.04%)	⬇️
presto	53.90% <100.00%> (+0.09%)	⬆️
python	83.37% <100.00%> (-0.03%)	⬇️
sqlite	77.92% <100.00%> (-0.05%)	⬇️
unit	54.39% <100.00%> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
superset/views/core.py	75.52% <100.00%> (-1.12%)	⬇️

... and 2 files with indirect coverage changes

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