libsnapshot: Remove misleading message

If we read up to a label, this error message is unneeded, and if we
don't, we already return an error message before this, leaving the old
message as entirely redundant and misleading.

Test: Run, verify "No COW Footer, recovered data" does not show in logs
Bug: 172026020
Change-Id: I31d054ccf898cf93c71ff201f0868e57cd1a6135