internal/imports: remove obsolete go:generate directive

CL 567837 deleted mkstdlib.go (a generator) and zstdlib.go (its output).

For golang/go#46136.
For golang/go#38706.

Change-Id: If5623e3ab014c2fee63b1d058c726ea5acb06aa2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/571276
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: dmitshur

internal/imports/imports.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:generate go run mkstdlib.go
-
 // Package imports implements a Go pretty-printer (like package "go/format")
 // that also adds or removes import statements as necessary.
 package imports