implement removeExportDirectory

Not yet called by Command.Export.

WebDAV needs this to clean up empty collections. Also, example.sh turned
out to not be cleaning up directories when removing content
from them, so it made sense for it to use this.

Remote.Directory did not need it, and since its cleanup method for empty
directories is more efficient than what Command.Export will need to do
to find empty directories, it uses Nothing so that extra work can be
avoided.

This commit was sponsored by Thom May on Patreon.

Remote/Helper/Export.hs

@@ -32,8 +32,9 @@ instance HasExportUnsupported (Annex (ExportActions Annex)) where
 			warning "store export is unsupported"
 			return False
 		, retrieveExport = \_ _ _ _ -> return False
-		, removeExport = \_ _ -> return False
 		, checkPresentExport = \_ _ -> return False
+		, removeExport = \_ _ -> return False
+		, removeExportDirectory = Just $ \_ -> return False
 		, renameExport = \_ _ _ -> return False
 		}