debug/gosym: remove outdated comment

Change-Id: I2bba13064c8d21ded41499c6ec225ef83d1a533e
Reviewed-on: https://go-review.googlesource.com/c/156997
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: cuonglm

src/debug/gosym/symtab.go

@@ -7,11 +7,6 @@
 // by the gc compilers.
 package gosym
 
-// The table format is a variant of the format used in Plan 9's a.out
-// format, documented at https://9p.io/magic/man2html/6/a.out.
-// The best reference for the differences between the Plan 9 format
-// and the Go format is the runtime source, specifically ../../runtime/symtab.c.
-
 import (
 	"bytes"
 	"encoding/binary"