Remove title attributes from social icons

[mrwweb]

The title attributes on those links are redundant with the screen reader text, don't work on touch devices, and generally aren't great. WordPress is moving toward removing all title attributes and Jetpack should do the same! If it's judged important that there should be a tooltip on the icons, then it should be introduced with a real, accessible tooltip function.

[jeherve]

Fixed in #5286