block: get rid of blk->guest_block_size

Commit 1b7fd729 ("block: rename buffer_alignment to
guest_block_size") noted:

  At this point, the field is set by the device emulation, but completely
  ignored by the block layer.

The last time the value of buffer_alignment/guest_block_size was
actually used was before commit 339064d5 ("block: Don't use guest
sector size for qemu_blockalign()").

This value has not been used since 2013. Get rid of it.

Cc: Xie Yongji <xieyongji@bytedance.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220518130945.2657905-1-stefanha@redhat.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Faria <afaria@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>