Remove deprecated includedFlags and excludedFlags from Holdout model
and simplify HoldoutConfig to treat all holdouts as global.

- Removed includedFlags and excludedFlags fields from Holdout
- Removed getIncludedFlags() and getExcludedFlags() methods
- Updated all 4 JSON parsers (GSON, Jackson, org.json, json-simple)
- Simplified HoldoutConfig logic (reduced from ~165 to ~85 lines)
- Removed 2 test methods testing deleted functionality
- Updated test JSON files and test utils

Verification: grep for includedFlags|excludedFlags returns 0 results
Net reduction: 355 lines (404 deletions, 49 additions)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>