Removed cross-compilation bobbins

Cross-compilation isn't working, and it's misleading to suggest it
does by having these here.

docker/Makefile

@@ -6,7 +6,7 @@ MAIN:=../
 all: docker-image
 
 weavedns: ../server/*.go ../main.go
-	go get -tags netgo $(MAIN) && GOARCH=amd64 GOOS=linux go build -tags netgo -ldflags '-linkmode external -extldflags "-static"' -o $@ -v $(MAIN)
+	go get -tags netgo $(MAIN) && go build -tags netgo -ldflags '-linkmode external -extldflags "-static"' -o $@ -v $(MAIN)
 
 .PHONY: docker-image