[rpc] Remove auth cookie on shutdown
[rpc] Remove auth cookie on shutdown

Accidentally removed in 40b556d3742a1f65d67e2d4c760d0b13fe8be5b7

PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX