Platforms: no highest-version upgrade dir on POSIX

As the Scalar.Service does not run the ProductUpgradeTimer on
macOS, and there is no Scalar.Service on Linux, and because
the UpgradeVerb does not record any highest-available version data
except on Windows, there is no necessity to implement the
GetUpgradeHighestAvailableVersionDirectory() method on POSIX.

We can therefore just return a NotImplementedException(),
and also simplify the GetUpgradeLogDirectoryParentDirectory()
method, which is used on POSIX to provide the path to the
log file generated by the "scalar upgrade" command.

Scalar.Common/Platforms/POSIX/POSIXPlatform.cs

@@ -189,12 +189,12 @@ public override string GetUpgradeProtectedDataDirectory()
 
         public override string GetUpgradeLogDirectoryParentDirectory()
         {
-            return this.GetUpgradeNonProtectedDirectory();
+            return Path.Combine(this.GetCommonAppDataRootForScalar(), ProductUpgraderInfo.UpgradeDirectoryName);
         }
 
         public override string GetUpgradeHighestAvailableVersionDirectory()
         {
-            return this.GetUpgradeNonProtectedDirectory();
+            throw new NotImplementedException();
         }
 
         public override Dictionary<string, string> GetPhysicalDiskInfo(string path, bool sizeStatsOnly)
@@ -392,11 +392,6 @@ private static bool TryGetEnvironmentVariable(string name, out string val, out s
             return true;
         }
 
-        private string GetUpgradeNonProtectedDirectory()
-        {
-            return Path.Combine(this.GetCommonAppDataRootForScalar(), ProductUpgraderInfo.UpgradeDirectoryName);
-        }
-
         private static class NativeMethods
         {
             public static string ResolveSymlink(string path, int maxPathLength)