Remove unused exceptions

Remove unused exceptions

The following exceptions are not used.
  class MigrationNotFound(NotFound):
  class BadDriverResponseStatus(VolumeDriverException):
  class InstanceNotFound(NotFound):
  class VolumeRetypeFailed(CinderException):
  class DuplicateSolidFireVolumeNames(SolidFireDriverException):

Change-Id: I891360f9c150eb8ea3f3968f12b9ad3da998981a