Administration: Prevent horizontal scrollbar in contextual help panel.

In [62145], an `::after` CSS rule was added that caused an overflow, resulting in an unintended scrollbar always appearing on Windows OS for example. This changeset removes the related CSS rule which is unnecessary to fix the initial issue. 

Follow-up to [62145].

Props wildworks, SergeyBiryukov, sabernhardt, audrasjb, huzaifaalmesbah, mehrazmorshed, mukesh27.
Fixes #64744.



git-svn-id: https://develop.svn.wordpress.org/trunk@62187 602fd350-edb4-49c9-b593-d223f7449a82