Layout: Remove double output of align-items in horizontal layouts

[andrewserong]

What?

Following on from vertical alignment being introduced to the layout support in #40013, this PR removes the double output of align-items for horizontal layouts.

Why?

It looks like we missed removing the existing align-items: center; output in the Layout support's server-rendering. While the "real" value takes precedence, it'd be good to tidy up the output so there's only a single property rendered. I've labelled this as a "bug", but it isn't at all a worrisome bug 😄

How?

• Remove the default align-items: center; output for horizontal layouts, given that the condition below it will always output an align-items value.

Testing Instructions

1. Add a Buttons block to a post
2. Add a couple of Buttons within the block, and use Shift+Enter to make one of the buttons go to multiple lines like the screenshot below, so that you can test that vertical alignment works.
3. Test the setting for vertical alignment, and save the post after each time and view on the front end of the site — make sure it still works (and that with no value, we still have a default value of center).

Screenshots or screencast

Before	After

[ntsekouras]

Nice! I saw that recently and forgot to make a patch 😄 . Thanks @andrewserong !