Clean up dead code: filter.

Clean up dead code: filter.

We haven't used this since at least landing the new SkPicture backend.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/f19385387d1b8f0b7e09f254979610d9858f474a

[mtklein_C, 2015-11-18 14:07:51]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-11-18 14:07:59]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1459583003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1459583003/1

[mtklein_C, 2015-11-18 14:13:08]

mtklein@chromium.org changed reviewers:
+ robertphillips@google.com

[commit-bot: I haz the power, 2015-11-18 14:19:38]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-18 14:19:39]

Dry run: This issue passed the CQ dry run.

[robertphillips, 2015-11-18 14:43:28]

lgtm

[mtklein, 2015-11-18 15:44:08]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-11-18 15:44:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1459583003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1459583003/1

[commit-bot: I haz the power, 2015-11-18 15:45:12]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f19385387d1b8f0b7e09f254979610d9858f474a