Fix typings for Batch

Tried using the typings with typescript 2.6/2.7 and neither worked. Both throw on `type Batch<K=any, V?=any>`. b353fb4 added the `?` before the default for some reason. AFAIK this is not valid typescript, and servers no purpose. Since `type Batch<K=any, V=any>` already has defaults, the values are optional. Without the question mark it seems to be working.

@vweevers can you elaborate on what you were trying to achieve?

index.d.ts

@@ -40,7 +40,7 @@ export interface AbstractIteratorOptions<K=any> {
   values?: boolean;
 }
 
-export type Batch<K=any, V?=any> = PutBatch<K, V> | DelBatch<K>
+export type Batch<K=any, V=any> = PutBatch<K, V> | DelBatch<K>
 
 export interface PutBatch<K=any, V=any> {
   type: 'put',