auth/credentials: Remove use of pytalloc_get_type() of NDR types in pycredentials

auth/credentials: Remove use of pytalloc_get_type() of NDR types in pycredentials

This function is based on a flawed premise that the
pointer is a talloc context, but the second element
in an array and any element in a structure is not a
talloc context.

The type has already been checked above.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jo Sutton <josutton@catalyst.net.nz>