RFR: 8278426: ImagePool uses terminally deprecated System.runFinalization method

[Lukasz Kostyra]

This change removes runFinalization calls in ImagePool::pruneCache method.

Finalizers are deprecated in JDK 18 and will be removed soon.

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Commit messages:
 - 8278426: Remove runFinalization calls in ImagePool::pruneCache

Changes: https://git.openjdk.org/jfx/pull/918/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=918&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278426
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/918.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/918/head:pull/918

PR: https://git.openjdk.org/jfx/pull/918