remove musl-dev in man/Dockerfile.armhf

Signed-off-by: Andrew Hsu <andrewhsu@docker.com>

man/Dockerfile.armhf

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y \
   bash \
   curl \
   gcc \
-  musl-dev \
   make
 
 ENV GO_VERSION 1.7.5