Delete panel plugins: flot, plotly, scatterplot, visx #1360

Job	Run time
setup-matrix	7s
run-integration-tests (./examples/app-with-backend)	3m 14s
run-integration-tests (./examples/panel-datalinks)	3m 7s
run-integration-tests (./examples/datasource-basic)	4m 14s
run-integration-tests (./examples/app-with-rbac)	1m 37s
run-integration-tests (./examples/app-with-scenes)	1m 32s
run-integration-tests (./examples/datasource-streaming-websocket/streaming-websocket-plugin)	1m 36s
run-integration-tests (./examples/app-basic)	2m 57s
run-integration-tests (./examples/panel-frame-select)	1m 28s
run-integration-tests (./examples/datasource-streaming-backend-websocket/streaming-backend-websoc...	1m 44s
run-integration-tests (./examples/datasource-http)	2m 56s
run-integration-tests (./examples/datasource-http-backend)	2m 11s
run-integration-tests (./examples/app-with-extensions)	2m 54s
run-integration-tests (./examples/app-with-extension-point)	3m 11s
run-integration-tests (./examples/app-with-service-account)	2m 39s
run-integration-tests (./examples/app-with-dashboards)	2m 50s
run-integration-tests (./examples/panel-basic)	2m 42s
run-integration-tests (./examples/datasource-logs)	1m 30s
notify	0s
	42m 29s

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