Block: remove widget CSS class from the block

[cabrerahector]

The block right now is adding a "widget" class to its main container.

Some themes however use this CSS class to style widgets / blocks and sometimes this causes some styling issues. For example, themes that use said class in the widget container element, then WPP "reapplies" said CSS rules to its own container causing problems like this (real life example reported by one user):

Let's remove that CSS class:

• Remove CSS class from widget block.
• Put a warning about this on the changelog, this change might affect people using .popular-posts.widget to style the block.