cmd/link: remove outdated cast and comment

This program is written in Go now.

Change-Id: Ieec21a1bcac7c7a59e88cd1e1359977659de1757
Reviewed-on: https://go-review.googlesource.com/28549Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/ld/data.go

@@ -586,13 +586,7 @@ func relocsym(ctxt *Link, s *Symbol) {
 				o += Symaddr(ctxt, r.Sym)
 			}
 
-			// NOTE: The (int32) cast on the next line works around a bug in Plan 9's 8c
-			// compiler. The expression s->value + r->off + r->siz is int32 + int32 +
-			// uchar, and Plan 9 8c incorrectly treats the expression as type uint32
-			// instead of int32, causing incorrect values when sign extended for adding
-			// to o. The bug only occurs on Plan 9, because this C program is compiled by
-			// the standard host compiler (gcc on most other systems).
-			o += r.Add - (s.Value + int64(r.Off) + int64(int32(r.Siz)))
+			o += r.Add - (s.Value + int64(r.Off) + int64(r.Siz))
 
 		case obj.R_SIZE:
 			o = r.Sym.Size + r.Add