app-usage: Remove crufty old "context"s idea

Back in the day, there was a proposed system of tracking apps in a
specific context.

The inspiration was that you may have used apps in multiple modes:
Firefox may have been used in both "Programmer Reference" and
"Kitten Videos" contexts. Early user response to the feedback wasn't
too positive - context switching is something that humans have trouble
doing implicitly, let alone explicitly. The old codebase still has a
few remnants of this around; let's finally put them to rest.

Note that we still write out a dummy context tag to the XML file - old
versions of the shell will flat out crash if you don't have one of those
in there, so just leave it in for compatibility sake.

https://bugzilla.gnome.org/show_bug.cgi?id=673767

src/shell-window-tracker.c

@@ -457,12 +457,6 @@ get_app_for_window (ShellWindowTracker    *tracker,
   return result;
 }
 
-const char *
-_shell_window_tracker_get_app_context (ShellWindowTracker *tracker, ShellApp *app)
-{
-  return "";
-}
-
 static void
 update_focus_app (ShellWindowTracker *self)
 {