appveyor.yml: remove npm-cache folder.
appveyor.yml: remove npm-cache folder.

mpc-hc/mpc-hc.org,mpc-hc/mpc-hc.org,FeodorFitsner/mpc-hc.org,mpc-hc/mpc-hc.org,FeodorFitsner/mpc-hc.org,FeodorFitsner/mpc-hc.org