Media: Hide image name overlay on gallery thumbnail row.

Set the generated content in the gallery selection strip to `content: none;` to remove the generated overlay. Follow up to [61757].

Props wildworks, hbhalodia, divyeshpatel01, huzaifaalmesbah, apermo, joedolson.
Fixes #64820.

git-svn-id: https://develop.svn.wordpress.org/trunk@61862 602fd350-edb4-49c9-b593-d223f7449a82