Since commit de336584d2 (package/docker-engine: split docker-{cli, engine},
bump to v18.09.0), the docker-engine package only builds the daemon part,
and the .mk file no longer use the _DAEMON option, so drop it.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Tested-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>