IB/core: Remove deprecated current_seq comments

current_seq was removed since the commit below.

Fixes: 36f30e48 ("IB/core: Improve ODP to use hmm_range_fault()")
Link: https://lore.kernel.org/r/20210823035246.3506-1-lizhijian@cn.fujitsu.com


Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>