Clean up or retire `DeploymentType`

[armiol]

We have a DeploymentType message:

enum DeploymentType {

    // Running in a standalone mode.
    //
    // The deployment is considered STANDALONE when no specific runtime is recognized. 
    //
    STANDALONE = 0;

    // Running on an App Engine local development server.
    APPENGINE_EMULATOR = 1;

    // Running on an App Engine cloud infrastructure.
    APPENGINE_CLOUD = 2;
}

At this point neither APPENGINE_EMULATOR, nor APPENGINE_CLOUD don't make much sense anymore.

In the real life, we are only targeting the STANDALONE mode for Spine 2.x.x, with AppEngine being out of scope. The primary reason the web browser integration via gRPC + grpc-web bridge (that is upcoming in the scope of Spine TS), and these technologies are not available on AppEngine.

The framework users may still use AppEngine via SPI extension points, but that's not what we will be targeting for deployment.