[Swift3] remove XCode warnings

[ahmet8282]

This fixes #6258 for Xcode 8.3.3

[wing328]

Tests passed via https://travis-ci.org/swagger-api/swagger-codegen/builds/263779486

cc @jaz-ah @jgavris @Edubits

[jaz-ah]

+1 looks fine @wing328

[wing328]

@jaz-ah Thanks for reviewing the change.

@ahmettahasakar thanks for the PR. When you've time, I wonder if you can make similar enhancement to the swift4 templates

cc @ehyche

[ahmet8282]

@wing328 I don't seem to see the same warnings with Xcode 9.0 beta. I see other warnings about @objc

The use of Swift 3 @objc inference in Swift 4 mode is deprecated. Please address deprecated @objc inference warnings, test your code with “Use of deprecated Swift 3 @objc inference” logging enabled, and disable Swift 3 @objc inference.

What should I do?

[ehyche]

@ahmettahasakar : agreed that we don't see the same compiler warnings in swift4 generated code. However, for the issue you noticed above regarding @objc infererence, I have fixed that in this PR:

#6468