xio: fix regression in authentication

xio: fix regression in authentication

The change I75a5dee2fd1f8cf45dbf416b0353736a43efd8e8 replaced
base64.encodestring(auth_key)[:-1] with
oslo_serialization.base64.encode_as_text(auth_key)[:-1]. It's wrong
because base64.encodestring() adds '\n' to the output, wheras
oslo_serialization.base64.encode_as_text() doesn't add '\n'.

This change removes '[:-1]' to strip the newline, it's no more
needed.

Partial-Implements: blueprint cinder-python3
Change-Id: Ib015a850d7cc2e1259518fecfe66c836002b28d9