Drop conditional when verifying entry checkpoint (#1917)

The log has been returning checkpoints for over a year now.
Additionally, there should be no persisted bundles where the inclusion
proof does not include a checkpoint, because it was marked as a required
field from its inception.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

Author: Hayden-IO

pkg/verify/verify.go

@@ -220,13 +220,9 @@ func VerifyLogEntry(ctx context.Context, e *models.LogEntryAnon, verifier signat
 		return err
 	}
 
-	// TODO: Add support for verifying consistency against an optional provided checkpoint.
-	// See https://github.com/sigstore/rekor/issues/988
-	// TODO: Remove conditional once checkpoint is always returned by server.
-	if e.Verification.InclusionProof.Checkpoint != nil {
-		if err := VerifyCheckpointSignature(e, verifier); err != nil {
-			return err
-		}
+	// Verify checkpoint, which includes a signed root hash.
+	if err := VerifyCheckpointSignature(e, verifier); err != nil {
+		return err
 	}
 
 	// Verify the Signed Entry Timestamp.