go/cfg: rm unused field

This was added by CL 555255 but is never used in there; probably a
leftover from an earlier code version.

Change-Id: I93fc6389577498c6bc09a155a4bfe6b9ecd627ca
Reviewed-on: https://go-review.googlesource.com/c/tools/+/702521
Commit-Queue: Alan Donovan <adonovan@google.com>
Auto-Submit: Kirill Kolyshkin <kolyshkin@gmail.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

Author: kolyshkin

go/cfg/cfg.go

@@ -53,7 +53,6 @@ import (
 //
 // The entry point is Blocks[0]; there may be multiple return blocks.
 type CFG struct {
-	fset   *token.FileSet
 	Blocks []*Block // block[0] is entry; order otherwise undefined
 }