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

tests/block_test_util.go

@@ -98,7 +98,7 @@ func (t *BlockTest) Run() error {
 	}
 
 	// import pre accounts & construct test genesis block & state root
-	db, _ := ethdb.NewMemDatabase()
+	db := ethdb.NewMemDatabase()
 	gblock, err := t.genesis(config).Commit(db)
 	if err != nil {
 		return err