Remove old Dockerfiles

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/Dockerfile.build.armhf

-FROM alexellis2/go-armhf:1.7.4
-
-RUN go get -d github.com/docker/docker/api/types \
-   && go get -d github.com/docker/docker/api/types/filters \
-   && go get -d github.com/docker/docker/api/types/swarm \
-   && go get -d github.com/docker/docker/client \
-   && go get github.com/gorilla/mux \
-   && go get github.com/prometheus/client_golang/prometheus
-RUN go get -d github.com/Sirupsen/logrus
-
-WORKDIR /go/src/github.com/alexellis/faas/gateway
-
-COPY metrics        metrics
-COPY requests       requests
-COPY tests          tests
-COPY handlers       handlers
-
-COPY server.go      .
-COPY types          types
-
-RUN go test -v ./tests && \
-    CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .