Remove title attributes: the Theme browser modal

[afercia]

See related #24766 and all the following tickets about title attributes.

When there's a Theme update available, the Theme browser modal displays a warning with a link that has a title attribute:

I'd propose to simply remove this title attribute: users already clicked on a Theme, they already know they're viewing details about "Theme xxx" (in fact, this is not the list of themes) and there's no need to specify the Theme name as context for the link.

[afercia]

The proposed patch also adds translator comments and removes a no more used variable, see [27748].

Side note: the HTML in the translatable strings should probably be removed but that should go in a separate ticket.

[afercia]

In 36015:

Accessibility: Remove title attributes from the Theme browser.

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

Fixes #35140.