feat(medusa): Simplify the transaction base service (#2007)

**What**
Simplify the transaction base service.

**How**

In fact, it does not need to be template and reduce the extensibility as the type is internally enforce. Now, the type is deduced by this which can be any derived class.

packages/medusa/src/services/auth.ts

@@ -16,7 +16,7 @@ type InjectedDependencies = {
  * Can authenticate a user based on email password combination
  * @extends BaseService
  */
-class AuthService extends TransactionBaseService<AuthService> {
+class AuthService extends TransactionBaseService {
   protected manager_: EntityManager
   protected transactionManager_: EntityManager | undefined
   protected readonly userService_: UserService