Remove's the plot sub-project.

People who need visualization with Tablesaw are encouraged to use the tablesaw-jsplot library instead. This layer is already better in many ways than the original. See the visualization section of the Tablesaw user guide for more information.

The native java plotting code was always something of a mess given the lack of a single, strong, appropriately licensed open-sourcevisualization library in Java. It relied on several other libraries to piece together its functionality, including Smile, XCharts, and JavaFx.

The JavaFx dependency was particularly concerning since it is large, and was moved to a separate module in Java 9, and removed from the JDK entirely in Java 11, presenting some additional complexity from a build/deployment standpoint across JDK versions.

Beyond that, the functionality of the library was very limited and both the API and the visual user interface were inconsistent due to the underlying inconsistencies between the libraries it was built on.