Edit Post "Tags" meta box: "Choose from the most used tags" result - CSS improvements

[tyxla]

While working on #31558 I've noticed that when clicking the Choose from the most used tags link, the result (the list of tags or the No tags found. message) needs a couple of visual improvements.

1. The box has rounded corners, which seems to be the leftover of an old WordPress version. I suggest that the rounded corners are removed, so this box is consistent with the flat design of the administration.
2. There is some margin to the bottom of the box, which is no longer needed, as the wrapping meta box already has enough margin. I suggest that this margin is removed for consistency.

[tyxla]

Choose from the most used tags result - removing rounded corners and bottom margin

[tyxla]

Attaching a before/after image.

[DrewAPicture]

I think we should probably keep the extra margin on the bottom to match some of the other core meta box styles, but +1 for the rounded border change. Patch still applies.

[SergeyBiryukov]

In 33742:

Remove rounded corners from "Choose from the most used tags" result in Tags meta box.

props tyxla.
fixes #31560.