ref_gl: remove complete useless macro, enable VBO under option

ref/gl/gl_rsurf.c

@@ -1792,11 +1792,7 @@ void R_GenerateVBO( void )
 	R_ClearVBO();
 
 	// we do not want to write vbo code that does not use multitexture
-#if ALLOW_VBO
-	if( !GL_Support( GL_ARB_VERTEX_BUFFER_OBJECT_EXT ) || !GL_Support( GL_ARB_MULTITEXTURE ) || glConfig.max_texture_units < 2 )
-#else
-	if( 1 )
-#endif
+	if( !GL_Support( GL_ARB_VERTEX_BUFFER_OBJECT_EXT ) || !GL_Support( GL_ARB_MULTITEXTURE ) || glConfig.max_texture_units < 2 || !gEngfuncs.Sys_CheckParm("-gl-allow-vbo-dontuse") )
 	{
 		gEngfuncs.Cvar_FullSet( "gl_vbo", "0", FCVAR_READ_ONLY );
 		return;