I can think of a couple methods
First, you can call the delete action file with AJAX. Once it completes successfully, remove the element from your current page. This is the fancy way, but may be difficult to pull off, as it looks like you may be somewhat new to this.
Second, you can send the context of your current page along to the delete file (the subFolderPath variable), add it to the cflocation when you redirect. It looks like you already have it with the url.folder variable, so you would do this:
<cflocation url="#buildUrl('main.files', "subfolderPath=#rc.folder")#" />
It looks like you're using FW/1, so I assumed 'buildURL()' works as advertised, and your url & form variables are in the rc
scope.
Once you arrive back on your initial page from the cflocation, you will want to put the selection back on the folder you had selected. In your loop that creates the <option>
tags, add a check to see if an option should be selected. I would usually do something like this:
<option value="..." <cfif listLast(rc.subFolderPath,"\") EQ name>selected</cfif>>#name#</option>