cmd/compile: remove unnecessary Xoffset assignment

In golang.org/cl/61130, I removed the need for setting Xoffset on
OXCASE Nodes, but missed this assignment.

Passes toolstash-check.

Change-Id: I90ab05add14981b89ee18e73e1cdf2f13e9f9934
Reviewed-on: https://go-review.googlesource.com/66934
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/cmd/compile/internal/gc/noder.go

@@ -1047,7 +1047,6 @@ func (p *noder) commClauses(clauses []*syntax.CommClause, rbrace src.Pos) []*Nod
 		if clause.Comm != nil {
 			n.List.Set1(p.stmt(clause.Comm))
 		}
-		n.Xoffset = int64(types.Block)
 		n.Nbody.Set(p.stmts(clause.Body))
 		nodes = append(nodes, n)
 	}