[swift4] Use RequestBuilder directly for RxSwift and PromiseKit

Created by: d-date

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

When we specify RxSwift or PromiseKit to ResponseAs in additional_properties, the generator will generate 3 methods, requestBuilder, default implementation and wrapped method.

We need only requestBuild and wrapped method so that reduce generated codes. This is not breaking change for users.

Before:

open class func addPet(pet: Pet) -> Observable<Void> {
        return Observable.create { observer -> Disposable in
            addPet(pet: pet) { data, error in
                if let error = error {
                    observer.on(.error(error))
                } else {
                    observer.on(.next(data!))
                }
                observer.on(.completed)
            }
            return Disposables.create()
        }
    }

After:

open class func addPet(pet: Pet) -> Observable<Void> {
        return Observable.create { observer -> Disposable in
            addPetWithRequestBuilder(pet: pet).execute { (response, error) -> Void in
                if let error = error {
                    observer.onError(error)
                } else {
                    observer.onNext(())
                }
                observer.onCompleted()
            }
            return Disposables.create()
        }
    }