[FSSDK-11090] remove unused plugins directory (#989) #983

Job	Run time
crossbrowser_and_umd_unit_tests	6h 0m 13s
lint	1m 3s
unit_tests (16)	1m 42s
unit_tests (18)	1m 27s
unit_tests (20)	1m 24s
unit_tests (22)	1m 22s
fullstack_production_suite / test	6s
integration_tests / test	5s
lint_markdown_files / lint	2m 20s
Coveralls coverage	3s
	6h 9m 45s

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