In order to delete the whole content of your database (and thus any demo content you might have imported) we suggest that you to install the Database Reset plugin.
You can also find the plugin by following this link.
Now you should install and activate this plugin.
As mentioned above, this will effectively delete the whole content from your database, not just the demo content. Therefore, please be sure you want to do this before proceeding with the steps that follow.
Now navigate to Tools > Database Reset, select all the tables and then click remove for the first two. Enter the security code you are provided with and click Reset Tables.