doc: remove outdated Makefile Fixes issue 7773.
doc: remove outdated Makefile
Fixes issue 7773.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/87400043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang