RFR: 8326612: Parallel: remove redundant assertion from ScavengeRootsTask

[Richard Reingruber]

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.

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Commit messages:
 - Remove redundant assertion

Changes: https://git.openjdk.org/jdk/pull/17994/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17994&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326612
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17994.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17994/head:pull/17994

PR: https://git.openjdk.org/jdk/pull/17994