we don't need a root_fbo function anymore

[skip ci]

common/headless_view.cpp

@@ -274,9 +274,5 @@ void HeadlessView::make_inactive() {
 
 void HeadlessView::swap() {}
 
-unsigned int HeadlessView::root_fbo() {
-    return fbo;
-}
-
 }
 

common/headless_view.hpp

@@ -34,7 +34,6 @@ public:
     void make_active();
     void make_inactive();
     void swap();
-    unsigned int root_fbo();
 
 private:
     void clear_buffers();

include/mbgl/map/view.hpp

@@ -39,12 +39,6 @@ public:
     // thread. This is called once just before the rendering thread terminates.
     virtual void make_inactive() = 0;
 
-    // Returns the base framebuffer object, if any, and 0 if using the system
-    // provided framebuffer.
-    virtual unsigned int root_fbo() {
-        return 0;
-    }
-
     virtual void notify() = 0;
 
     // Notifies a watcher of map x/y/scale/rotation changes.