go.image/vp8: remove the filter look-up tables.

The benchmarks are a little slower, but the code is shorter.

benchmark                          old ns/op     new ns/op     delta
BenchmarkDecodeVP8SimpleFilter     621950        644184        +3.57%
BenchmarkDecodeVP8NormalFilter     4045745       4269431       +5.53%

LGTM=r
R=r, minux, ruiu
CC=golang-codereviews
https://golang.org/cl/108140045
1 file changed