Revert Chrome image rendering tweak

The Chrome bug appears to have been fixed:
https://bugs.chromium.org/p/chromium/issues/detail?id=562162

This CSS rule makes images look very ugly in Safari, so removing it.

ref #1259

framework/core/less/common/App.less

@@ -171,8 +171,6 @@
 .Header-logo {
   max-height: 30px;
   vertical-align: middle;
-  // Prevent blurriness in Chrome
-  image-rendering: -webkit-optimize-contrast;
 }
 
 // On phones, the header is displayed inside of the drawer. We lay its

framework/core/less/common/Avatar.less

@@ -14,8 +14,6 @@
     height: 100%;
     border-radius: 100%;
     vertical-align: top;
-    // Prevent blurriness in Chrome
-    image-rendering: -webkit-optimize-contrast;
   }
 }