Toolbar: A11y: Fix color contrast for zero comments.

Remove the 50% opacity change for when the adminbar comment indicators has zero comments.

Props louischan, hbhalodia, sabernhardt, joedolson.
Fixes #64811.

git-svn-id: https://develop.svn.wordpress.org/trunk@61861 602fd350-edb4-49c9-b593-d223f7449a82