Above is a great 3 minute video tutorial demonstrating how to get rid of extraneous or superfluous linework that REVIT adds in to triangular flat sloped roofs and floors.
You use 'invisible' linework to cover up the unwanted lines. To reverse the process do the same thing but change the linework back to (by catagory) by selecting (by catagory. . . . or any other linetype) from he linework type selector
Note the before and after pictures below.