RFR: 8342462: TextAreaSkin: remove USE_MULTIPLE_NODES

[Andy Goryachev]

Removed "not yet fully implemented" USE_MULTIPLE_NODES and related code.

I would like to remove the early unfinished idea of using multiple Text nodes in TextAreaSkin to clean up the code, to make it easier to do fixes for [JDK-8342233](https://bugs.openjdk.org/browse/JDK-8342233) and [JDK-8296266](https://bugs.openjdk.org/browse/JDK-8296266).

Also some minor cleanup.

## Summary of Changes

- removed USE_MULTIPLE_NODES and code paths that correspond to its `true` value
- permanently adding one Text node to `paragraphNodes` Group, keeping the latter for compatibility purposes
- removed any code that scans `paragraphNodes` children
- using `getTextNode()` in place of `((Text)paragraphNodes.getChildren().get(0))`

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Commit messages:
 - 8342462 TextAreaSkin: remove USE_MULTIPLE_NODES

Changes: https://git.openjdk.org/jfx/pull/1601/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1601&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342462
  Stats: 270 lines in 1 file changed: 26 ins; 172 del; 72 mod
  Patch: https://git.openjdk.org/jfx/pull/1601.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1601/head:pull/1601

PR: https://git.openjdk.org/jfx/pull/1601