Remove legacy source build property usage (#43435)

eng/restore-toolset.ps1

@@ -10,7 +10,7 @@ function InitializeCustomSDKToolset {
 
   # The following frameworks and tools are used only for testing.
   # Do not attempt to install them in source build.
-  if ($env:DotNetBuildFromSource -eq "true" -or $productBuild -or $properties -like "*DotNetBuildRepo=true*") {
+  if ($productBuild -or $properties -like "*DotNetBuildRepo=true*") {
     return
   }