Main project ignores CS0618

Due to the fact that .ToHashSet() is not presented in netstandard
but presented in actual runtime it we had to obsolete our version
and now we have some warnings for legitimate usage of our version

Author: alex-kulakov

Orm/Xtensive.Orm/Xtensive.Orm.csproj

@@ -14,6 +14,7 @@
     <AssemblyOriginatorKeyFile>$(OrmKeyFile)</AssemblyOriginatorKeyFile>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <WarningLevel>2</WarningLevel>
+	<NoWarn>$(NoWarn);CS0618</NoWarn>
   </PropertyGroup>
   <PropertyGroup Label="Debug" Condition="'$(Configuration)'=='Debug'">
     <DefineConstants>TRACE;DEBUG;NETSTANDARD</DefineConstants>