Revert "all: use fixdocs tool to store package documentation in doc.go"

This reverts commit 00292b2d57.

diff/fd/doc.go

@@ -1,8 +0,0 @@
-// Copyright ©2017 The gonum Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-/*
-Package fd provides functions to approximate derivatives using finite differences.
-*/
-package fd // import "gonum.org/v1/gonum/diff/fd"