[Optional] Simplify driver and OpenNI.ini path resolution

As described in the previous commit, resolving the driver path based on environment variable (OPENNI2_DRIVERS_PATH) and reading OpenNI.ini from the current directory could be only confusing if we started to resolve it based on OpenNI2.dll's directory. If you agree, please use this commit to remove these features. It will make the behavior and the code much simpler and more explainable.