Accessibility: Remove title attributes from the Theme browser.

Also, adds translators comments and removes a no more used variable, see [27748].

Fixes #35140.
Built from https://develop.svn.wordpress.org/trunk@36015


git-svn-id: https://core.svn.wordpress.org/trunk@35980 1a063a9b-81f0-0310-95a4-ce76da25c4cd