Convert openvr_init from class to function

Since the OpenVR initialization code is no longer inside the
overlaycontroller there is no need for it to be available during object
construction. Therefore it doesn't make sense to have it be an object.

The shutdown call was completely removed because shutdowns are handled
by an OpenVR event that calls the relevant shutdown code. The destructor
for the old object was never called, removing it adds clarity to the
actual operation of the code and also removes unnecessary code.

src/main.cpp

@@ -37,7 +37,7 @@ int main( int argc, char* argv[] )
                                    commandLineArgs.forceRemoveManifest );
     }
 
-    openvr_init::OpenVR_Init openvr_init(
+    openvr_init::initializeOpenVR(
         openvr_init::OpenVrInitializationType::Overlay );
 
     try