osmesa: tidy up renderbuffer refCount initialisation

32141e53d1 changed _mesa_init_renderbuffer() to set it to 1 for
us.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/mesa/drivers/osmesa/osmesa.c

@@ -549,7 +549,6 @@ new_osmesa_renderbuffer(struct gl_context *ctx, GLenum format, GLenum type)
       _mesa_init_renderbuffer(&srb->Base, name);
 
       srb->Base.ClassID = OSMESA_RENDERBUFFER_CLASS;
-      srb->Base.RefCount = 1;
       srb->Base.Delete = osmesa_delete_renderbuffer;
       srb->Base.AllocStorage = osmesa_renderbuffer_storage;