RFR: Remove assertions constraining card marking for reference processing

[William Kemper]

Shenandoah does _referent_ based processing for weak reference processing. We may see a mix of young and old references on the discovered list. We therefore need to mark cards for old references linked on the discovered list to young references to satisfy remembered set verification. These references may be mixed in any collection cycle.

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Commit messages:
 - Remove assertions constraining card marking for reference processing

Changes: https://git.openjdk.org/shenandoah/pull/241/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=241&range=00
  Stats: 16 lines in 1 file changed: 0 ins; 10 del; 6 mod
  Patch: https://git.openjdk.org/shenandoah/pull/241.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/241/head:pull/241

PR: https://git.openjdk.org/shenandoah/pull/241