[Sync] Remove DropAttachments from AttachmentService and SyncData

[Sync] Remove DropAttachments from AttachmentService and SyncData

AttachmentService and SyncData no longer need to provide a drop method
beecause model type code can drop attachments using its AttachmentStore.

BUG=

Committed: https://crrev.com/f9648891191bb34bce0a4c4d0110d82dc23b9ee6
Cr-Commit-Position: refs/heads/master@{#318981}

[maniscalco, 2015-03-02 19:46:14]

Pavel, would you please review this change?

[maniscalco, 2015-03-03 17:23:22]

maniscalco@chromium.org changed reviewers:
+ pavely@chromium.org

[maniscalco, 2015-03-03 17:23:23]

Hi Pavel, would you please review this change?

[pavely, 2015-03-03 19:11:26]

lgtm

[maniscalco, 2015-03-03 23:39:41]

The CQ bit was checked by maniscalco@chromium.org

[commit-bot: I haz the power, 2015-03-03 23:40:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/973513002/20001

[commit-bot: I haz the power, 2015-03-04 00:47:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-04 00:48:51]

Patchset 2 (id:??) landed as
https://crrev.com/f9648891191bb34bce0a4c4d0110d82dc23b9ee6
Cr-Commit-Position: refs/heads/master@{#318981}