Remove `content-l__main` class #10799

Job	Run time
backend (lint)	1m 28s
backend (validate-migrations)	1m 33s
backend (validate-assets)	2m 9s
backend (validate-translations)	1m 26s
backend (unittest)	5m 50s
backend (unittest-django42)	5m 46s
coverage	1m 46s
	19m 58s

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