Add test for no-frame SkyCoord repr
Use NoFrame for SkyCoord with no specified frame
Expunge frame_name attribute in favor of frame.name
Added preferred_attr_units to NoFrame and reverted test changes that it makes unnecessary

at @taldcroft's suggestion
Merge pull request #2500 from eteq/fix-2486-take2

Alternative fix for showing "no frame" in SkyCoord