Remove unprefixed and unused fadeIn animation

fixes #1115

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

@@ -23,19 +23,3 @@
   border: 0;
   color: inherit;
 }
-
-// Animations
-
-@keyframes fadeIn {
-  0% {
-    opacity: 0;
-  }
-
-  25% {
-    opacity: 1;
-  }
-
-  100% {
-    opacity: 0;
-  }
-}