When debugging code, you would come across instances, where you need to attach external sources (jars/zips) in order to continue debugging process with external library sources in place. However, unless you attach these sources through project structure option, you will find it hard to remove such a source attachment or to reattach a new source over it.
The solution for this is to remove the referred resource from it's current path. Since this source is not within the project it is just referring to the file which is located externally. So in order to remove this attached source, you have to remove the original external source file from its location.