RFR: 8350916: Remove misleading warning "Cannot dump shared archive while using shared archive"

[Ioi Lam]

Please review this trivial change. I replaced an unreachable `warning()` call with an assert and an explanation in comments.

I verified with running all CDS tests and the assert is never triggered.

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Commit messages:
 - Remove misleading warning "Cannot dump shared archive while using shared archive"

Changes: https://git.openjdk.org/jdk/pull/23836/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23836&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350916
  Stats: 10 lines in 1 file changed: 7 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23836.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23836/head:pull/23836

PR: https://git.openjdk.org/jdk/pull/23836