go.text/encoding/simplifiedchinese: remove redundant if check.
go.text/encoding/simplifiedchinese: remove redundant if check.

The improvement is barely noticible, but it surely can't hurt.

benchmark               old MB/s     new MB/s  speedup
BenchmarkGBKEncoder       181.94       182.25    1.00x

R=r
CC=golang-dev
https://codereview.appspot.com/13244047

bryanxu/go-zh.text,d0f/go-zh.text,webfd/go-zh.text,scalingdata/go.text,ssor/normGo