docker: don't specify datadir in lnd container

In this commit, we fix a lingering issue in the execution 
of the lnd container, after the new macaroon based 
authentication was added. With the new authentication 
feature, if the datadir was changed, but `lncli` wasn't 
updated to point to the macaroon path, then none of 
the commands would work. 

To fix this, we simply omit setting the data directory.

docker/lnd/start-lnd.sh

@@ -46,7 +46,6 @@ NETWORK=$(set_default "$NETWORK" "simnet")
 CHAIN=$(set_default "$CHAIN" "bitcoin")
 
 lnd \
-    --datadir="/data" \
     --logdir="/data" \
     "--$CHAIN.rpccert"="/rpc/rpc.cert" \
     "--$CHAIN.active" \