UBIFS: remove unused functions

Remove 'xent_key_init_hash()' and 'data_key_init_flash()' functions,
as they are unot used anywhere.

Signed-off-by: Subrata Modak <subrata@linux.vnet.ibm.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>