Administration: Correct checkbox width in list tables on smaller screens.

Previously, checkboxes for pending comments were partially hidden due to an added `4px` border.

Props audrasjb, jeremyfelt, talldanwp, melchoyce, marcelo2605, waleedt93.
Fixes #48815.
Built from https://develop.svn.wordpress.org/trunk@46845


git-svn-id: https://core.svn.wordpress.org/trunk@46645 1a063a9b-81f0-0310-95a4-ce76da25c4cd