Remove Gantt charts in version 3.0

[kolchfa-aws]

Gantt charts are removed in version 3.0 per opensearch-project/dashboards-visualizations#430

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[kolchfa-aws]

@joshuali925 Could you please review this PR?

[joshuali925]

thanks. should this also be removed?

documentation-website/_install-and-configure/install-dashboards/plugins.md

Lines 124 to 130 in 63a1a17

	#### Gantt Chart OpenSearch Dashboards
	```bash
	sudo bin/opensearch-dashboards-plugin install https://d3g5vo6xdbdb9a.cloudfront.net/downloads/opensearch-dashboards-plugins/opensearch-gantt-chart/opensearchGanttChartOpenSearch Dashboards-{{site.opensearch_major_minor_version}}.0.0.zip
	```
	This plugin adds a new Gantt chart visualization.

[kolchfa-aws]

This text is inside the comment so it's not visible on the site. But I removed the table entry for the Gantt chart plugin.

[natebower]

LGTM