RFR: 8326612: Parallel: remove redundant assertion from ScavengeRootsTask

[Albert Mingkun Yang]

On Sat, 24 Feb 2024 09:29:20 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

> This change removes a redundant assertion from `ScavengeRootsTask`.
> The assertion is redundant because `_old_gen` is initialized to `old_gen` at L296 which is dereferenced at L297 before the assertion is reached.

Marked as reviewed by ayang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17994#pullrequestreview-1908458207