libsnapshot: Remove OnlineKernelSnapshotWriter.

This was never used or tested. The idea was to unify update_engine's
write paths on ISnapshotWriter. But given that legacy VAB is "legacy",
it doesn't make sense to begin refactoring that code and potentially
introducing bugs. Let's just remove this instead.

Bug: 280529365
Test: builds
Change-Id: Ie2f531bd140e183dfde4b65a144f3b4acc28e78a