Remove system-ui from the font-stack since it's buggy

Reason: https://infinnie.github.io/blog/2017/systemui.html

packages/@uppy/core/src/_variables.scss

@@ -1,5 +1,5 @@
 // Fonts
-$font-family-base: system-ui, -apple-system, BlinkMacSystemFont, Segoe UI, Roboto,
+$font-family-base: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto,
                    Oxygen, Ubuntu, Cantarell, Fira Sans, Droid Sans, Helvetica Neue, sans-serif !default;
 
 // Colors