Remove GLU header from OpenGL 2 renderer

We don't seem to be actually using it.

Backends/RmlUi_Backend_X11_GL2.cpp

@@ -34,7 +34,6 @@
 #include <RmlUi/Core/Context.h>
 #include <GL/gl.h>
 #include <GL/glext.h>
-#include <GL/glu.h>
 #include <GL/glx.h>
 #include <X11/Xlib.h>
 #include <X11/cursorfont.h>

Backends/RmlUi_Renderer_GL2.cpp

@@ -36,17 +36,14 @@
 #if defined RMLUI_PLATFORM_WIN32
 	#include "RmlUi_Include_Windows.h"
 	#include <gl/Gl.h>
-	#include <gl/Glu.h>
 #elif defined RMLUI_PLATFORM_MACOSX
 	#include <AGL/agl.h>
 	#include <OpenGL/gl.h>
 	#include <OpenGL/glext.h>
-	#include <OpenGL/glu.h>
 #elif defined RMLUI_PLATFORM_UNIX
 	#include "RmlUi_Include_Xlib.h"
 	#include <GL/gl.h>
 	#include <GL/glext.h>
-	#include <GL/glu.h>
 	#include <GL/glx.h>
 #endif
 

Samples/shell/src/RendererExtensions.cpp

@@ -35,17 +35,14 @@
 	#if defined RMLUI_PLATFORM_WIN32
 		#include <RmlUi_Include_Windows.h>
 		#include <gl/Gl.h>
-		#include <gl/Glu.h>
 	#elif defined RMLUI_PLATFORM_MACOSX
 		#include <AGL/agl.h>
 		#include <OpenGL/gl.h>
 		#include <OpenGL/glext.h>
-		#include <OpenGL/glu.h>
 	#elif defined RMLUI_PLATFORM_UNIX
 		#include <RmlUi_Include_Xlib.h>
 		#include <GL/gl.h>
 		#include <GL/glext.h>
-		#include <GL/glu.h>
 		#include <GL/glx.h>
 	#endif