filechooser: Remove default size machinery

filechooser: Remove default size machinery

The FileChooser ToolKit (fctk) had its own machinery to handle default
sizes which was completely busted and trying to marshal random numbers
through the widget hierarchy that maybe made sense in 2012 but don't do
now.

Get rid of it, just keep the dialog's GSetting - which funnily enough
used to be written by the dialog but written by the widget.
But that's fctk for you.