Remove NOT_REACHED() call in brave_domains

[nvonpentz]

Resolves

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Test Plan:

[cdesouza-chromium]

I think this should be NOTREACHED actually. We own the enum, which is

enum ServicesEnvironment { DEV, STAGING, PROD };

I particularly think this function should be something like:

std::string ConvertEnvironmentToString(brave_domains::ServicesEnvironment env) {
 switch(env) {
    case ServicesEnvironment::DEV:
       return kBraveServicesSwitchValueDev;
    case ServicesEnvironment::STAGING:
       return kBraveServicesSwitchValueStaging;
    case ServicesEnvironment::PROD:
       return kBraveServicesSwitchValueProduction;  
 }
  NOTREACHED_NORETURN();
}

This way we don't get some hidden bug if a new key is added to brave_domains::ServicesEnvironment.

[cdesouza-chromium]

You don't need to do it if you don't feel like, but ideally we should also change this enum from

enum ServicesEnvironment { DEV, STAGING, PROD };

to:

enum class ServicesEnvironment { kDev, kStaging, kProd };

So, if you feel like doing that too....