RFR: 8342462: TextAreaSkin: remove USE_MULTIPLE_NODES [v2]

[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))`

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  wrong comments

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1601/files
  - new: https://git.openjdk.org/jfx/pull/1601/files/5780b280..a3523279

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1601&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1601&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 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