Remove unused variable in OgreRenderingEngine.cc (#1119)

Signed-off-by: Shameek Ganguly <[email protected]>
(cherry picked from commit 414735f)

Author: shameekganguly

ogre/src/OgreRenderEngine.cc

@@ -802,8 +802,6 @@ void OgreRenderEngine::InitAttempt()
 
   this->initialized = false;
 
-  Ogre::ColourValue ambient;
-
   /// Create a dummy rendering context.
   /// This will allow gz-rendering to run headless. And it also allows OGRE to
   /// initialize properly