channeldb: remove redudant test
channeldb: remove redudant test

After addition of the bidirectional payment test in htlcswitch packet,
which is more abstract, we may remove redudant test.

AndrewSamokhvalov/lnd,LightningNetwork/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,ddustin/lnd,aakselrod/lnd