Excises the descriptive metadata datastream.

Gets rid of the descriptive metadata datastream.  It's no longer assigned to/read from anywhere, and has been cut from the NuCoreFile specification.  Also updated all of our read methods in metadata_assignment.rb to try to read from the MODS datastream first and then fall over to the DC stream in the event that it's not present.  This more clearly represents our intentions w.r.t the significance of each datastream.