Customize: Extra arrows in widgets list

[dlh]

​As spotted by @melchoyce, there are unnecessary arrows appearing with each widget in the available widgets list in the Customizer.

From what I can tell, this behavior began with the removal of a CSS rule in [42794].

ticket:40677#comment:12 seems to suggest that the changes in that commit weren't intended to affect the Customizer. So, the attached patch takes the brute-force method of just adding the removed rule to customize-widgets.css.

There might be a more subtle way to fix the problem, or I might be misunderstanding the intent of the original commit, so I'm open to other ideas.

[westonruter]

In 43244:

Customize: Hide expansion arrows in Customizer's available widgets list.

Fixes regression introduced by [42794].

Props dlh.
See #40677.
Fixes #43983.

[audrasjb]

Reopening for 4.9.9 consideration as seen during accessibility team bug-scrub meeting.