Remove default case in mc_downstream.
Remove default case in mc_downstream.

It runs in the client process and would eat the client's messages and
produce an error occasionally, causing, for example, tgen to stop.

ceejatec/ns_server,daverigby/ns_server,ingenthr/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,vzasade/ns_server,nimishzynga/ns_server,ingenthr/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,vzasade/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,daverigby/ns_server,daverigby/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,ceejatec/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,membase/ns_server,mschoch/ns_server,vzasade/ns_server,nimishzynga/ns_server,membase/ns_server,ingenthr/ns_server,ingenthr/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server