Removed types for generate method

closes #38

Since 2.0.0 the `generate` method has been removed from the ObjectID
class, this updates the types to reflect that.

refs a518e352

objectid.d.ts

@@ -33,7 +33,5 @@ declare interface ObjectIDCtor {
   createFromHexString(hexString: string): ObjectID;
   isValid(hexString: string): boolean;
   isValid(ObjectID: ObjectID): boolean;
-  generate(): string;
-  generate(time: number): string;
   toString(): string;
 }