all: get rid of error when creating memory database (#16716)

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition

core/chain_indexer_test.go

@@ -48,7 +48,7 @@ func TestChainIndexerWithChildren(t *testing.T) {
 // multiple backends. The section size and required confirmation count parameters
 // are randomized.
 func testChainIndexer(t *testing.T, count int) {
-	db, _ := ethdb.NewMemDatabase()
+	db := ethdb.NewMemDatabase()
 	defer db.Close()
 
 	// Create a chain of indexers and ensure they all report empty