In order to disable authentication for a specific application in Oracle Apex, you need to simply create a new authentication schema from the preconfigured list called "No Authentication (using DAD)". Here is the reference from the documentation.
1) Click the Application Builder icon
2) Select the application you wish to disable logging in for
3) Click Shared Components
4) Under Security, select Authentication Schemes
5) Click Create
6) Verify that "Based on a pre-configured scheme from the gallery" is selected and click Next
7) Select "No Authentication (using DAD)" from the Scheme Type select list; enter a name (such as "No authentication" and click next.
8) The authentication scheme that you created has no been activated:
Name: Jose Gustavo
Creation Date: 2016-01-11