[JavaScript] Remove default parameters syntax, as it's an ES6 feature

[f-f]

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Description of the PR

This removes some syntax for "default parameters" from the JavaScript client. The reason is that it's a ES6 feature that doesn't work on IE11

cc @CodeNinjai @frol @cliffano

[f-f]

@wing328 thank you!

[wing328]

Thanks again for the PR, which has been included in the v4.0.2 release: https://twitter.com/oas_generator/status/1141610197766426626