[Smell] JavaModelManager.shutdown() join on Job family that seems unused

While working on #1190 and investigating the behavior of JavaModelManager, I noticed that its `shutdown` method calls `JobManager.join(JavaCore.PLUGIN_ID,...)` to "wait for the initialization job to finish". However, it looks like nowhere in JDT Core a job is created with a `belongsTo` that makes it part of the `JavaCore.PLUGIN_ID`. So it looks like this `join` is a no-op and could be removed; or that an important job misses instructions to be part of this `JavaCore.PLUGIN_ID` family.

Start with JavaModelManager.shutdown() and trace its JobManager.join(JavaCore.PLUGIN_ID, ...) call, then inspect JDT Core jobs for belongsTo usage with that family. Determine whether the join is unused or whether an initialization job is missing family membership; done means documenting and applying the resolved cleanup or correction.

Written by the indexing model from the issue text.