What?

This PR splits out the flex-shrink removal from #46139, in case it's desirable to land that change before dealing with the flex-basis default value, which is being discussed in that PR.

Why?

The change in #46139 has some nuance surrounding what to do with pre-populating flex-basis values, so it might be easier to try to land the flex-shrink removal first. Removing flex-shrink was discussed earlier over in: #45364 (comment)

How?

• Remove flex-shrink rule from layout.php and layout.js when outputting the flexSize value.

Testing Instructions

1. Add a Row block to a post and add a couple of child blocks in it;
2. Select a child block and under "Dimensions" in the sidebar select "Width";
3. Set width to "fixed", add a fixed size e.g. 500px in the input, save;
4. Verify that on a small viewport the fixed width block shrinks and doesn't overflow it.

Screenshots or screencast

Desktop (block is set to 500px)	Mobile (block shrinks to fill available space)