Cleanup NonWindowsGetDomainName() (#6948)

Add an optional extended description…

src/System.Management.Automation/CoreCLR/CorePsPlatform.cs

@@ -545,22 +545,6 @@ namespace System.Management.Automation
             return Unix.NativeMethods.SetDate(&tm) == 0;
         }
 
-        internal static string NonWindowsGetDomainName()
-        {
-            string name = Unix.NativeMethods.GetFullyQualifiedName();
-            if (!string.IsNullOrEmpty(name))
-            {
-                // name is hostname.domainname, so extract domainname
-                int index = name.IndexOf('.');
-                if (index >= 0)
-                {
-                    return name.Substring(index + 1);
-                }
-            }
-            // if the domain name could not be found, do not throw, just return empty
-            return string.Empty;
-        }
-
         // Hostname in this context seems to be the FQDN
         internal static string NonWindowsGetHostName()
         {

test/csharp/test_CorePsPlatform.cs

@@ -86,29 +86,6 @@ namespace PSTests.Parallel
             }
         }
 
-        [Fact]
-        public static void TestGetDomainName()
-        {
-            var startInfo = new ProcessStartInfo
-            {
-                FileName = @"/usr/bin/env",
-                Arguments = "dnsdomainname",
-                RedirectStandardOutput = true,
-                UseShellExecute = false
-            };
-            using (Process process = Process.Start(startInfo))
-            {
-                 // Get output of call to hostname without trailing newline
-                string domainName = process.StandardOutput.ReadToEnd().Trim();
-                process.WaitForExit();
-
-                // The process should return an exit code of 0 on success
-                Assert.Equal(0, process.ExitCode);
-                // It should be the same as what our platform code returns
-                Assert.Equal(domainName, Platform.NonWindowsGetDomainName());
-            }
-        }
-
         [Fact]
         public static void TestIsExecutable()
         {