ImagePool uses terminally deprecated System.runFinalization method

The System.runFinalization and Runtime.runFinalization methods are terminally deprecated by JEP 421 (JDK-8274609). ImagePool.pruneCache calls System.runFinalization which results in a "removal" warning.

$ gradle --info -PLINT=removal sdk
...
modules/javafx.graphics/src/main/java/com/sun/scenario/effect/impl/ImagePool.java:259: warning: [removal] runFinalization() in System has been deprecated and marked for removal
        System.runFinalization();
              ^
modules/javafx.graphics/src/main/java/com/sun/scenario/effect/impl/ImagePool.java:261: warning: [removal] runFinalization() in System has been deprecated and marked for removal
        System.runFinalization();
              ^

The only purpose of calling runFinalization was to cause finalize to be called on JavaFX images. Since the JavaFX runtime no longer uses finalization anywhere, the calls to runFinalization are unnecessary and should be removed.

NOTE: several tests also call System.runFinalization; I will file a separate bug for the tests.