Follow steps below to easily disable App Signing from Google Play Console :-
Step 1 : Go to Google Play Console
Step 2 : If you have a Google account, login, else Create an Account
Step 3 : Click on Create Application
Step 4 : Select Language and enter Title then click on create button
Step 5 : Enter details
Step 6 : Click on Save draft.After creation of the app now click on the "App releases"
Step 7 : Click on the MANAGE in Production track
Step 8 : Click on the "CREATE RELEASE"
Step 9 : Here you see "Google Play App Signing" dialog, click on the "OPT-OUT" button.
Step 10 : Click on the I'M SURE, OPT-OUT button.
Step 11 : Here you can see App Signing is disabled , Now you are ready to Drop your APK file with your signing key after that click on Review button.