Remove unimplemented Utility cmdlets from Unix

ConvertFrom-SddlString and Unblock-File are Windows-specific cmdlets
with no equivalent on Unix.

squash! Remove Unblock-File from Unix

test/powershell/Modules/Microsoft.PowerShell.Utility/Unimplemented-Cmdlet.Tests.ps1

@@ -0,0 +1,13 @@
+Describe "Unimplemented Utility Cmdlet Tests" -Tags "CI" {
+
+    $Commands = @(
+        "Unblock-File",
+        "ConvertFrom-SddlString"
+    )
+
+    foreach ($Command in $Commands) {
+        It "$Command should only be available on Windows" {
+            [bool](Get-Command $Command -ErrorAction SilentlyContinue) | Should Be $IsWindows
+        }
+    }
+}