go/consensus/cometbft/light: Only fetch from light store for now

In practice the previously introduced fetch from light client caused
the light client to fall back to slow backwards verification due to
target blocks being in the past, below the pruning window.

.changelog/5481.bugfix.md

@@ -0,0 +1,5 @@
+go/consensus/cometbft/light: Only fetch from light store for now
+
+In practice the previously introduced fetch from light client caused
+the light client to fall back to slow backwards verification due to
+target blocks being in the past, below the pruning window.

go/consensus/cometbft/light/service.go

@@ -289,21 +289,17 @@ func (c *client) GetLightBlock(ctx context.Context, height int64) (*consensus.Li
 		return lb, rpc.NewNopPeerFeedback(), nil
 	}
 
-	// Light client.
-	lightClientSource := func() (*consensus.LightBlock, rpc.PeerFeedback, error) {
-		clb, err := c.lc.GetVerifiedLightBlock(ctx, height)
+	// Light client store.
+	lightClientStoreSource := func() (*consensus.LightBlock, rpc.PeerFeedback, error) {
+		lb, err := c.GetStoredLightBlock(height)
 		if err != nil {
-			c.logger.Debug("failed to fetch light block from light client",
+			c.logger.Debug("failed to fetch light block from light client store",
 				"err", err,
 				"height", height,
 			)
 			return nil, nil, err
 		}
 
-		lb, err := api.NewLightBlock(clb)
-		if err != nil {
-			return nil, nil, err
-		}
 		return lb, rpc.NewNopPeerFeedback(), nil
 	}
 
@@ -316,7 +312,7 @@ func (c *client) GetLightBlock(ctx context.Context, height int64) (*consensus.Li
 	var mergedErr error
 	for _, src := range []func() (*consensus.LightBlock, rpc.PeerFeedback, error){
 		localBackendSource,
-		lightClientSource,
+		lightClientStoreSource,
 		directPeerQuerySource,
 	} {
 		lb, pf, err := src()