include/block.h: remove outdated comment

include/block.h: remove outdated comment

There are a couple of errors in bdrv_drained_begin header comment:
- block_job_pause does not exist anymore, it has been replaced
  with job_pause in b15de82867
- job_pause is automatically invoked as a .drained_begin callback
  (child_job_drained_begin) by the child_job BdrvChildClass struct
  in blockjob.c. So no additional pause should be required.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210903113800.59970-1-eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>