internal/gcimporter: update comment

Remove a reference to
  GOROOT/src/go/internal/gcimporter/iimport.go
This file no longer exists.

Change-Id: Ia5e012b0c39a71390167a00f0890b7492d789188
Reviewed-on: https://go-review.googlesource.com/c/tools/+/635798
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Tim King <taking@google.com>

internal/gcimporter/iimport.go

@@ -5,8 +5,6 @@
 // Indexed package import.
 // See iexport.go for the export data format.
 
-// This file is a copy of $GOROOT/src/go/internal/gcimporter/iimport.go.
-
 package gcimporter
 
 import (