Prevent default OpenGL context creation in wxGLCanvas under macOS

This renders unnecessary the previous workarounds for using NSOpenGLView
as the base class for wxGLCanvas view, so revert them.

See:

- 2ab430965c which introduced the use of
  NSOpenGLView (see https://github.com/wxWidgets/wxWidgets/pull/846).
- ea68934b8e for the first workaround due
  to the use of NSOpenGLView and its default context creation.
- 6974a6ceaa for the refinement of the
  workaround above.
- e6ae83c386 and
  b822e9efe8 which tweaked it futher.

This commit renders unnecessary all the changes since the initial switch
to NSOpenGLView by just preventing NSOpenGLView from creating its own
context and thus enforcing the use of wxGLContext associated with
wxGLCanvas.

Closes https://github.com/wxWidgets/wxWidgets/pull/1617

src/osx/cocoa/glcanvas.mm

@@ -150,6 +150,13 @@ WXGLPixelFormat WXGLChoosePixelFormat(const int *GLAttrs,
         impl->doCommandBySelector(aSelector, self, _cmd);
 }
 
+- (NSOpenGLContext *) openGLContext
+{
+    // Prevent the NSOpenGLView from making it's own context
+    // We want to force using wxGLContexts
+    return NULL;
+}
+
 @end
 
 bool wxGLCanvas::DoCreate(wxWindow *parent,
@@ -201,24 +208,6 @@ bool wxGLContext::SetCurrent(const wxGLCanvas& win) const
 
     [m_glContext makeCurrentContext];
 
-    // the missing redraw upon resize problem only happens when linked against 10.14
-#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_14
-    // At least under macOS 10.14.5 we need to do this in order to update the
-    // context with the new size information after the window is resized.
-    if ( WX_IS_MACOS_AVAILABLE_FULL(10, 14, 5) )
-    {
-        if ( WX_IS_MACOS_AVAILABLE(10, 15) )
-        {
-            // no workaround needed under 10.15 anymore
-        }
-        else
-        {
-            NSOpenGLView *v = (NSOpenGLView *)win.GetHandle();
-            [v setOpenGLContext: m_glContext];
-        }
-    }
-#endif
-        
     return true;
 }