statistics: remove experimental extended stats (stats_extended) #65882
Description
Summary
Remove the experimental extended stats feature (tidb_enable_extended_stats / STATS_EXTENDED / mysql.stats_extended).
Background
- Feature is hidden and gated by tidb_enable_extended_stats (default OFF).
- Only correlation stats are actually computed; cardinality/dependency are rejected.
- Ongoing maintenance cost across parser/DDL/executor/stats/planner/tests.
Proposal (remove)
- Keep sysvar and parser grammar/keywords.
- Remove DDL/executor handlers.
- Remove extended stats storage, cache, build, GC, JSON dump.
- Remove mysql.stats_extended table creation (decide upgrade handling).
- Remove tests and warnings referencing extended stats.
Compatibility / upgrade
- Decide whether to drop mysql.stats_extended or keep unused on upgrade.
- Confirm planner behavior without extended‑stats correlation adjustment.
Acceptance criteria
- No references to mysql.stats_extended or ExtendedStats in codebase.
- Tests updated and green.