Rename 'delete_application' to 'unregister_application' in clipper_admin

[withsmilo]

• related issue : Suggestion - API scheme improvement #603

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[withsmilo]

Jenkins ok to test

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[simon-mo]

@withsmilo can you still create a new method named delete_application and call unregister_application underneath. So existing code doesn’t need to face this breaking change.

In delete application, we can even throw a warning saying this api is deprecated and is aliasing unregistered application

[withsmilo]

@simon-mo , I understand it. It's great idea! I will refactor this PR as soon as possible.

[rkooo567]

For me, it is good to go if merge conflict is resolved

[withsmilo]

@rkooo567 : I resolved the merge conflict, and added an warning message to the old API(delete_application) reflecting @simon-mo 's opnion.

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[withsmilo]

Jenkins test this please

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[rkooo567]

I think it is good to go. I will just approve because this PR is simple enough. @simon-mo

[withsmilo]

@rkooo567 : Thanks! I will merge this if @simon-mo confirms

[withsmilo]

@simon-mo Thanks! 😀