Fix visual regression of the donate button

plugins/CoreHome/stylesheets/_donate.less

@@ -72,7 +72,8 @@
 
 .piwik-donate-call .donate-submit input {
     margin-left: 13px !important;
-    border-style: none;
+    border-style: none !important;
+    box-shadow: none !important;
     background-image: none;
     padding: 0 !important;
 }