Help/About: Correct header spacing on shorter screens after removing the musician name.

Remove some unnecessary CSS.

Follow-up to [48730].

Props ryelle.
Reviewed by whyisjake, SergeyBiryukov.
Merges [48736] to the 5.5 branch.
See #50416.
Built from https://develop.svn.wordpress.org/branches/5.5@48737


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48499 1a063a9b-81f0-0310-95a4-ce76da25c4cd