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/genesis_test.go

@@ -141,7 +141,7 @@ func TestSetupGenesis(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		db, _ := ethdb.NewMemDatabase()
+		db := ethdb.NewMemDatabase()
 		config, hash, err := test.fn(db)
 		// Check the return values.
 		if !reflect.DeepEqual(err, test.wantErr) {