libsnapshot: Remove consistency check after merge

Merge thread will mark merge-completion after msync on header is
complete. This should be definitive enough to track the completion
status.

Bug: 305187301
Test: OTA on Pixel
Change-Id: I366dc5052fa91a6eacf394a1970200cdebc0e135
Signed-off-by: Akilesh Kailash <akailash@google.com>