nvme-pci: remove nvme_disable_admin_queue

nvme_disable_admin_queue has only a single caller, and just calls two
other funtions, so remove it to clean up the remove path a little more.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>