RFR: 8283675: Line not removed from LineChart when series cleared

[Karthik P K]

The issue is present in AreaChart along with the LineChart. Issue is fixed in both the charts as part of this PR.
The line elements in case of Line chart and both line element and fill element in the case of Area charts were not cleared in `makePaths()` method in AreaChart.java. Hence the line and fill area were not getting cleared when series was cleared.

Made changes in code to clear line element and fill element as required in the `makePaths()` method.

Added tests to validate the changes in both LineChart and AreaChart.

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Commit messages:
 - Line chart issue fix

Changes: https://git.openjdk.org/jfx/pull/1214/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1214&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283675
  Stats: 46 lines in 3 files changed: 39 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1214.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1214/head:pull/1214

PR: https://git.openjdk.org/jfx/pull/1214