Remove DevTools store references to legacy analytics

https://github.com/flutter/devtools/issues/9765 tracks removing legacy analytics. To clean up DevTools storage files, we will leave logic to remove the key that used to track legacy analytics. Remove this logic after 1 yeare (April 2027).

Read issue #9765 first to understand the legacy analytics removal and its timing. Then locate the DevTools storage files containing the key-removal logic and remove that logic after April 2027; done means the obsolete cleanup no longer remains while the rest of storage behavior is preserved.

索引モデルが issue の本文から書いたものです。