include/sysemu/blockdev.h: remove drive_get_max_devs

Remove drive_get_max_devs, as it is not used by anyone.

Last use was removed in commit 8f2d75e8
("hw: Drop superfluous special checks for orphaned -drive").

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20211215121140.456939-4-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>