statistics: deprecate enable-stats-cache-mem-quota and remove the non-quota stats cache path

`enable-stats-cache-mem-quota` defaults to `true`, but we still keep the old non-quota stats cache path.

When the flag is `true`, stats cache uses the `lfu` backend and updates the cache in place.
When the flag is `false`, it uses the old `mapcache` backend and updates the cache through `CopyAndUpdate`.

The non-quota path is already effectively legacy code. There is even a TODO saying this branch should be removed because quota will always be enabled.

Besides being extra maintenance burden, this old path also has a correctness issue: `CopyAndUpdate` does not respect `UpdateOptions.SkipMoveForward`, while `Update` does.

This matters for the targeted stats refresh after `ANALYZE`. That refresh sets `SkipMoveForward = true`, so it should not move the cache watermark forward. But if `enable-stats-cache-mem-quota = false`, the COW path may still advance `maxTblStatsVer`. After that, the next incremental `stats_meta` scan may skip some delta rows from other tables, leaving their stats stale until they are analyzed again.

The default config is not affected because quota is enabled by default.

I think we should deprecate this flag and remove the legacy path instead of fixing dead code.

Proposed changes:

* Keep parsing `enable-stats-cache-mem-quota`, but always treat it as `true`.
* Log a warning if users set it to `false`.
* Remove the `mapcache` branch in `NewStatsCache`.
* Remove `CopyAndUpdate` and the old branch in `UpdateStatsCache`.
* Remove the `internal/mapcache` package if there are no other users.
* Update tests and benchmarks that still set this flag to `false`.