Do not use backup file for external metadata

Do not use backup file for external metadata

When external metatdata handler supports manage_reshape()
and recover_backup() functions in super switch backup file is not required
and can be omitted. For backup purposes metadata specific mechanisms
are used.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: Krzysztof Wojcik <krzysztof.wojcik@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>