blockdev: no need to drain+flush in hmp_drive_del

bdrv_close already does that, and in fact hmp_drive_del would need
another drain after the flush (which bdrv_close does).  So remove
the duplication.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1432822629-25401-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>