sample/mdev/mbochs: remove set but not used variable 'mdev_state'

Fixes gcc '-Wunused-but-set-variable' warning:

samples/vfio-mdev/mbochs.c: In function mbochs_ioctl:
samples/vfio-mdev/mbochs.c:1188:21: warning: variable mdev_state set but not used [-Wunused-but-set-variable]

It's not used any more since commit 104c7405

 ("vfio:
add edid support to mbochs sample driver")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>