Cleanup: GPUContext: Remove default_framebuffer_

This is now handled by FrameBuffer wrapping.

source/blender/gpu/intern/gpu_context.cc

@@ -127,12 +127,6 @@ GLuint GPU_vao_default(void)
   return static_cast<GLContext *>(active_ctx)->default_vao_;
 }
 
-GLuint GPU_framebuffer_default(void)
-{
-  BLI_assert(active_ctx); /* need at least an active context */
-  return static_cast<GLContext *>(active_ctx)->default_framebuffer_;
-}
-
 GLuint GPU_vao_alloc(void)
 {
   GLuint new_vao_id = 0;

source/blender/gpu/intern/gpu_context_private.hh

@@ -79,7 +79,6 @@ struct GPUContext {
 };
 
 GLuint GPU_vao_default(void);
-GLuint GPU_framebuffer_default(void);
 
 /* These require a gl ctx bound. */
 GLuint GPU_buf_alloc(void);

source/blender/gpu/opengl/gl_context.cc

@@ -46,10 +46,6 @@ using namespace blender::gpu;
 GLContext::GLContext(void *ghost_window, GLSharedOrphanLists &shared_orphan_list)
     : shared_orphan_list_(shared_orphan_list)
 {
-  default_framebuffer_ = ghost_window ?
-                             GHOST_GetDefaultOpenGLFramebuffer((GHOST_WindowHandle)ghost_window) :
-                             0;
-
   glGenVertexArrays(1, &default_vao_);
 
   float data[4] = {0.0f, 0.0f, 0.0f, 1.0f};

source/blender/gpu/opengl/gl_context.hh

@@ -63,8 +63,6 @@ class GLContext : public GPUContext {
  public:
   /** Default VAO for procedural draw calls. */
   GLuint default_vao_;
-  /** Default framebuffer object for some GL implementation. */
-  GLuint default_framebuffer_;
   /** VBO for missing vertex attrib binding. Avoid undefined behavior on some implementation. */
   GLuint default_attr_vbo_;
   /**