Even installing .NET 4 continues to add this folder. There seems to be no permanent way to delete this folder beyond its deletion after registering .NET using IIS or after installing a new version of the .NET Framework.
Some people say that they throw out the read-only attribute in the directory, but ... which still supports the directory.
See my answer to a similar question. Prevent adding an aspnet_client folder to .NET sites
Repeat this answer:
Check out the MSDN documentation for the ASP.NET IIS Registration Tool (Aspnet_regiis.exe) . There is an e and ea command line switch that you should use to clear it after the fact.
You can use Aspnet_regiis.exe to install and uninstall the system client side script, for example a script for client-side validation. Use the -c option to install the client script for the version of ASP.NET associated with this tool. (The script is installed in the Aspnet_client subdirectory of each IIS site directory.) To remove the client side of the script only for the version of ASP.NET associated with this tool, use the -e option. To remove the client side of the script for all installed versions of ASP.NET, use the -ea option.
(Emphasis added.)