RFR: 8295026: Remove unused fields in StyleSheet

[Andrey Turbanov]

There are 3 unused fields in single `StyleSheet` file:
1. `static final Border noBorder = new EmptyBorder(0,0,0,0);`
2. `static final int DEFAULT_FONT_SIZE = 3;`
3. `URL imageurl;` in nested `javax.swing.text.html.StyleSheet.ListPainter` class

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Commit messages:
 - [PATCH] Remove unused fields in StyleSheet

Changes: https://git.openjdk.org/jdk/pull/10523/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10523&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295026
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10523.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10523/head:pull/10523

PR: https://git.openjdk.org/jdk/pull/10523