Removes all references of Microsoft.Web.Helpers - we don't need to ship this, we don't reference it and it has compatibility problems. Also updates the version to 7.3.0

src/Umbraco.Web.UI/Views/Web.config

@@ -20,7 +20,6 @@
         <add namespace="Umbraco.Core" />
         <add namespace="Umbraco.Core.Models" />
         <add namespace="Umbraco.Web.Mvc" />
-        <add namespace="Microsoft.Web.Helpers" />
         <add namespace="umbraco" />
         <add namespace="Examine" />
       </namespaces>