block-backend: Remove deadcode

block-backend: Remove deadcode

blk_by_public last use was removed in 2017 by
  c61791fc23 ("block: add aio_context field in ThrottleGroupMember")

blk_activate last use was removed earlier this year by
  eef0bae3a7 ("migration: Remove block migration")

blk_add_insert_bs_notifier, blk_op_block_all, blk_op_unblock_all
last uses were removed in 2016 by
  ef8875b549 ("virtio-scsi: Remove op blocker for dataplane")

blk_iostatus_disable last use was removed in 2016 by
  66a0fae438 ("blockjob: Don't touch BDS iostatus")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>