clean_duplicates() should remove *_files folder when .html files are removed

[cderv]

While trying to test a workflow for my Hugo Apero workshop to help convert over an existing academic site, clean_duplicates() was used. Because of the choice I made with syntax highlighters, the testing implies converting over a site (where the posts have no need for pandoc) to full markdown mode.

.html are the duplicates in this case as the new post output will be .md so clean_duplicates() now correctly helps remove HTML file but their companion *_files folder still remain.

They should be removed too

_Originally posted by @apreshill in #629

[yihui]

The *_files directory cannot be deleted if it contains other subdirectories such as figure-html. For now, I'm only deleting the header-attrs subdirectory. After that, if the *_files directory becomes empty, I'll delete it, otherwise it won't be deleted. I think this should solve the original problem. Thanks!