test: remove TestHasBlockReturnsNil

The function body is just
```go
  return nil
```
And it's testing that this code returns nil.
Pointless


This commit was moved from ipfs/go-ipfs-exchange-offline@cdbe3d1

exchange/offline/offline_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"testing"
 
-	blocks "github.com/ipfs/go-block-format"
 	cid "github.com/ipfs/go-cid"
 	ds "github.com/ipfs/go-datastore"
 	ds_sync "github.com/ipfs/go-datastore/sync"
@@ -23,22 +22,6 @@ func TestBlockReturnsErr(t *testing.T) {
 	t.Fail()
 }
 
-func TestHasBlockReturnsNil(t *testing.T) {
-	store := bstore()
-	ex := Exchange(store)
-	block := blocks.NewBlock([]byte("data"))
-
-	// we don't need to do that for the test, but that illustrate the normal workflow
-	if err := store.Put(context.Background(), block); err != nil {
-		t.Fatal(err)
-	}
-
-	err := ex.NotifyNewBlocks(context.Background(), block)
-	if err != nil {
-		t.Fail()
-	}
-}
-
 func TestGetBlocks(t *testing.T) {
 	store := bstore()
 	ex := Exchange(store)