Remove `isReturnDefaultValues` to debug easy.

[Jaehwa-Noh]

What I have done and why

This project seems doesn't need to use isReturnDefaultValues. All test had been passed as no used isReturnDefaultValues. isReturnDefaultValues is not recommended to use, because it made hard to debug. documentation

[!Caution] Take care when setting the returnDefaultValues property to true. The null/zero return values can introduce regressions to your tests, which are hard to debug and might allow failing tests to pass. Only use it as a last resort.