docs: Remove developers.txt

Everything in there has either been removed in the past or is outdated
information.

docs/developers.txt

@@ -1,60 +0,0 @@
-Things to care about when using/programing for GTK
-==================================================
-
-This file is meant to collect some frequently triggered failures when
-programming for/with Gtk, having the spirit of a developers FAQ.
-It is also the correct place to list up things that programmers should
-care about in general.
-
-In the hope that this text might be useful to someone,
-
-        - Tim Janik <timj@gimp.org>
-          1998/02/11
-
-
-Automatic destruction of widgets on removal from parent
--------------------------------------------------------
-
-This is a reference counting issue, you would want to refer
-to refcounting.txt on it.
-
-
-What are all the widget flags about?
-------------------------------------
-
-Refer to the file widget_system.txt which covers widget flags and the
-resulting invariants in a detailed way.
-
-
-GdkSurface pointers may be NULL in GdkEvents
--------------------------------------------
-
-The notification nature of the signal mechanism might cause events to
-be emitted that have their GdkSurface pointer set to NULL.
-This is due to the fact that certain events need to be emitted after the
-real GdkSurface of a widget is not any longer pertinent.
-It's up to the signal handling function (application) to check for the
-window field of the event structure to be != NULL, if it is going to
-perform any operations through Gdk calls on it.
-Events that a likely to trigger a missing check for the window pointer
-currently are (and correspond to the trailing signals):
-
-GDK_SELECTION_CLEAR     GtkWidget::selection_clear_event
-GDK_FOCUS_CHANGE        GtkWidget::focus_in_event
-                        GtkWidget::focus_out_event
-
-Events that are assured to have a valid GdkEvent.any.surface field are
-
-GDK_EXPOSE		GtkWidget::expose_event
-
-
-Writing Gdk functions
----------------------
-
-When writing Gdk functions that operate on GdkSurface structures in any
-meaningful sense, that is casting to a GdkSurfacePrivate structure for
-access to fields other then GdkSurface.user_data, the programmer is
-recommended to check for the GdkSurfacePrivate.destroyed field to be ==
-FALSE, especially if the GdkSurfacePrivate.xwindow field is used.
-Silent abortion of the Gdk function is the correct behaviour if this
-condition isn't met.