remove unnecessary border radius from modal body

nobody seems to be able to remember why it was added in the first place
and it is preventing full width body content

add changelog

Author: robin-drexler

UNRELEASED.md

@@ -20,6 +20,7 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 
 ### Bug fixes
 
+- Removed unnecessary border-radius from `Modal` body ([#1584](https://github.com/Shopify/polaris-react/pull/1584))
 - Fixed inconsistent width depending on your browser/version in `Sheet` ([#1569](https://github.com/Shopify/polaris-react/pull/1569))
 - Fixed text and other elements from being selected in Safari when dragging the color picker ([#1562](https://github.com/Shopify/polaris-react/pull/1562))
 - Fixed `Banner` `title` overflowing when set to a single long word ([#1553](https://github.com/Shopify/polaris-react/pull/1553))

src/components/Modal/Modal.scss

@@ -3,7 +3,6 @@ $small-width: rem(620px);
 .BodyWrapper {
   display: flex;
   flex-grow: 1;
-  border-radius: border-radius(large);
   overflow-x: hidden;
   -webkit-overflow-scrolling: touch;
 }