doc: remove outdated Makefile

Fixes #7773.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/87400043

doc/Makefile

-# Copyright 2009 The Go Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style
-# license that can be found in the LICENSE file.
-
-RAWHTML=\
-	articles/defer_panic_recover.rawhtml\
-	articles/error_handling.rawhtml\
-	articles/slices_usage_and_internals.rawhtml\
-	articles/laws_of_reflection.rawhtml\
-	articles/c_go_cgo.rawhtml\
-	articles/concurrency_patterns.rawhtml\
-	articles/godoc_documenting_go_code.rawhtml\
-	articles/gobs_of_data.rawhtml\
-	articles/json_and_go.rawhtml\
-	articles/json_rpc_tale_of_interfaces.rawhtml\
-	articles/image_draw.rawhtml\
-	articles/image_package.rawhtml\
-	effective_go.rawhtml\
-	go1.rawhtml\
-
-all: $(RAWHTML)
-
-%.rawhtml: %.html
-	godoc -url /doc/$< >$@
-
-clean:
-	rm -f $(RAWHTML)
-
-compare:
-	for i in $(RAWHTML); do \
-		godoc -url /doc/$${i/.rawhtml/.html} | diff -u $$i -; \
-	done