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-app-system.c

@@ -386,8 +386,7 @@ _shell_app_system_notify_app_state_changed (ShellAppSystem *self,
  * @self: A #ShellAppSystem
  *
  * Returns the set of applications which currently have at least one
- * open window in the given context.  The returned list will be sorted
- * by shell_app_compare().
+ * open window.  The returned list will be sorted by shell_app_compare().
  *
  * Returns: (element-type ShellApp) (transfer container): Active applications
  */