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

[mScott224]

On Fri, 28 Feb 2025 02:10:16 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> 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.

Marked as reviewed by mScott224 at github.com (no known OpenJDK username).

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PR Review: https://git.openjdk.org/jdk/pull/23836#pullrequestreview-2649966352