Use built-in symbol

refs: https://github.com/RogueException/Discord.Net/pull/1043#issuecomment-385223999

src/Discord.Net.Core/Discord.Net.Core.csproj

@@ -6,7 +6,6 @@
     <Description>The core components for the Discord.Net library.</Description>
     <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net45;netstandard1.1;netstandard1.3;netstandard2.0</TargetFrameworks>
     <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.1;netstandard1.3;netstandard2.0</TargetFrameworks>
-    <DefineConstants Condition=" '$(TargetFramework)' == 'netstandard2.0' ">NETSTANDARD2</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="10.0.2" />

src/Discord.Net.Core/Entities/Roles/Color.cs

@@ -1,6 +1,6 @@
 using System;
 using System.Diagnostics;
-#if NETSTANDARD2
+#if NETSTANDARD2_0 || NET45
 using StandardColor = System.Drawing.Color;
 #endif
 
@@ -100,7 +100,7 @@ namespace Discord
                 (uint)(b * 255.0f);
         }
 
-#if NETSTANDARD2
+#if NETSTANDARD2_0 || NET45
         public static implicit operator StandardColor(Color color) =>
             StandardColor.FromArgb((int)color.RawValue);
         public static explicit operator Color(StandardColor color) =>