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

consensus/clique/snapshot_test.go

@@ -352,7 +352,7 @@ func TestVoting(t *testing.T) {
 			copy(genesis.ExtraData[extraVanity+j*common.AddressLength:], signer[:])
 		}
 		// Create a pristine blockchain with the genesis injected
-		db, _ := ethdb.NewMemDatabase()
+		db := ethdb.NewMemDatabase()
 		genesis.Commit(db)
 
 		// Assemble a chain of headers from the cast votes