Stop adding the Corax search engine setting to existing databases

[ramonsmits]

UpdateDatabaseSettings added Indexing.Auto.SearchEngineType and Indexing.Static.SearchEngineType with value Corax to every database that had no such settings, at every start-up, including a start in maintenance mode. The intent was a one-time pin for databases created before 6.20. The code cannot tell "never pinned" from "the operator removed the setting", so it also undid an operator's choice.

RavenDB Studio removes a database setting whose value equals the server default. An operator who sets both keys to Lucene on a server whose default is Lucene therefore deletes the keys, and ServiceControl restored Corax at the next start. The "Migrating the whole database" procedure in the documentation produced exactly this.

The pin had no benefit. RavenDB stores the engine in each index and reads it back when the database opens, so a database default does not rebuild existing indexes. The default is used only for a new index, a reset, or a side-by-side replacement after a definition change.

• Both DatabaseSetup classes no longer write the Corax settings. New databases are still created with Lucene.
• DatabaseSettingsTests deletes the settings, reloads the database, runs the setup and asserts the settings stay absent.

Existing settings written by 6.20.0 to 6.21.2 are not removed; the next PR in the stack makes the engine of rebuilt indexes independent of them.