new: Remove workaround for detecting newly created directory objects

Previously, notmuch_database_get_directory did not indicate whether or
not the returned directory object was newly created, which required a
workaround to distinguish newly created directory objects with no
child messages from directory objects that had no mtime set but did
have child messages.  Now that notmuch_database_get_directory
distinguishes whether or not the directory object exists in the
database, this workaround is no longer necessary.