RFR: 8251555: Remove unused focusedWindow field in glass Window to avoid leak

[Kevin Rushforth]

This is a follow-up to [JDK-8241840](https://bugs.openjdk.java.net/browse/JDK-8241840) that removes an unused static
`focusedWindow` field and its associated getter and setter from the platform-independent glass Window class. This is
entirely unused by any of the glass platforms. The Monocle platform keeps track of the `focusedWindow` attribute itself.

The existing `test.javafx.stage.FocusedWindowNativeTest` automated test, which was added as part of
[JDK-8241840](https://bugs.openjdk.java.net/browse/JDK-8241840), was failing fairly consistently on one of our test
machines before this fix, and now passes.

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Commit messages:
 - Remove unused focusedWindow field in glass Window to avoid leak

Changes: https://git.openjdk.java.net/jfx/pull/286/files
 Webrev: https://webrevs.openjdk.java.net/jfx/286/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251555
  Stats: 15 lines in 1 file changed: 0 ins; 15 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/286.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/286/head:pull/286

PR: https://git.openjdk.java.net/jfx/pull/286