Admin: Remove border radius on focus state in admin sidebar.

Match the `:hover` and `:focus` states in the admin sidebar, restoring the sharp-edged border used prior to the admin reskin. Remove the 2px border radius from focused admin sidebar links.

Props juanfra, audrasjb, niravsherasiya7707, khushi1501, shailu25, joedolson.
Fixes #64860.

git-svn-id: https://develop.svn.wordpress.org/trunk@62108 602fd350-edb4-49c9-b593-d223f7449a82