I don't know how many SFTP APIs you were considering in your review, but removing a file is a very simple operation and any API which supports that operation would be suitable. Looking for the "best" at this task isn't a good use of anybody's time.
There is no single command in version 3 of the SFTP protocol (which is what openssh conforms to) for removing the content of a directory so that any Java library that does this in one command will internally need to first list all the files in the directory and then iterate over the list removing each one in turn. This should take no more than half a dozen lines of code for you to write yourself and unless the Java library does the deletes asynchronously (which is what the protocol allows) your code will be as fast as any other.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com