[web] Remove extraneous border-radius property in modal.css: modalContainer

Summary:
This is getting overridden by the border-radius: 8px below so it isn't having any effect.

The second instance is from 6 days ago and the first is from 3 months ago, so guessing the border-radius from 6 days ago "is the right one."

Test Plan:

1. Put code editor and modal side by side
2. Comment out first border-radius property
3. Check that appearance of modal doesn't change
4. Manually change the border-radius to 15px to make sure I would have noticed the visual change

Reviewers: def-au1t, palys-swm, benschac, ashoat

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3891