Remove `html_root_url` setting. #607

Job	Run time
test	50s
msrv	1m 14s
check	5s
check format	9s
check (--no-default-features)	4s
check (--no-default-features --features use_alloc)	25s
check (--all-targets --all-features)	16s
All checks succeeded	1s
	3m 4s

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