fix: adding support for net35 net462 and netstandard2.1

Author: sbansla

src/Twilio/Http/SystemNetHttpClient.cs

@@ -107,22 +107,7 @@ private HttpRequestMessage BuildHttpRequest(Request request)
             string helperLibVersion = AssemblyInfomation.AssemblyInformationalVersion;
 
             string osName = "Unknown";
-#if !NETSTANDARD1_4
             osName = Environment.OSVersion.Platform.ToString();
-#else       
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
-            {
-                osName = "Windows";
-            }
-            else if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
-            {
-                osName = "MacOS";
-            }
-            else if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
-            {
-                osName = "Linux";
-            }
-#endif
 
             string osArch;
 #if !NET462