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-usage.h

@@ -13,10 +13,8 @@ G_DECLARE_FINAL_TYPE (ShellAppUsage, shell_app_usage,
 
 ShellAppUsage* shell_app_usage_get_default(void);
 
-GSList *shell_app_usage_get_most_used (ShellAppUsage *usage,
-                                       const char    *context);
+GSList *shell_app_usage_get_most_used (ShellAppUsage *usage);
 int shell_app_usage_compare (ShellAppUsage *self,
-                             const char    *context,
                              const char    *id_a,
                              const char    *id_b);