Use Marshal.ZeroFreeCoTaskMemUnicode

This removes the ClrFacade function of the same name as this API is in
.NET Core 1.0 Marhsal class, not SecureStringMarshal.

src/Microsoft.PowerShell.LocalAccounts/LocalAccounts/Extensions.cs

@@ -160,7 +160,7 @@ namespace System.Management.Automation.SecurityAccountsManager.Extensions
 #if CORECLR
             IntPtr buffer = SecureStringMarshal.SecureStringToCoTaskMemUnicode(str);
             string clear = Marshal.PtrToStringUni(buffer);
-            SecureStringMarshal.ZeroFreeCoTaskMemUnicode(buffer);
+            Marshal.ZeroFreeCoTaskMemUnicode(buffer);
 #else
             var bstr = Marshal.SecureStringToBSTR(str);
             string clear = Marshal.PtrToStringAuto(bstr);
@@ -200,4 +200,4 @@ namespace System.Management.Automation.SecurityAccountsManager.Extensions
             return ex.MakeErrorRecord(ex.ErrorName, ex.ErrorCategory, target ?? ex.Target);
         }
     }
-}
+}