RFR: 8295323: Unnecessary HashTable usage in StyleSheet

[Phil Race]

On Sat, 1 Oct 2022 16:53:17 GMT, Andrey Turbanov <aturbanov at openjdk.org> wrote:

> Hashtable was used only from single thread. And only non-null key/values added to it. We can safely replace it with HashSet.

So I think this is OK, but please be sure you have run at least the related Swing jtreg tests

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Marked as reviewed by prr (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10522