DataBase: Delete BigText class

Python 34 env failed with:
  > ...
  > TypeError: Cannot create a consistent method resolution
  > order (MRO) for bases TDComparator, Comparator

Full traceback: http://paste.openstack.org/show/151305/

This patch fixes this issue.

Change-Id: Ib7df62278e1121c457468467e04012a58411cf1d
Merge "DataBase: Delete BigText class"