verity: remove write_cb

Now that there are no users we can remove the write_cb infrastructure.

BUG=chromium-os:19952
TEST=Ran unit tests from dm-verity.git.
     Ran platform_DMVerityCorruption and platform_DMVerityBitCorruption.
TESTED_ON=Alex

Change-Id: I1614ff03385769dfac477f80c4a31e3787910322
Reviewed-on: http://gerrit.chromium.org/gerrit/7103
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Tested-by: Mandeep Singh Baines <msb@chromium.org>
3 files changed