shell: Revert "Let panels have their own commandline flags"

This reverts commit 31a8a99440.

This was meant for bgo#695885 which has stalled for a while, so this
feature has no in-tree user. This commit removes it for now, this can be
readded when users for it materialize.

https://bugzilla.gnome.org/show_bug.cgi?id=751597

shell/cc-panel-loader.h

@@ -30,8 +30,6 @@ G_BEGIN_DECLS
 
 void     cc_panel_loader_fill_model     (CcShellModel  *model);
 GList   *cc_panel_loader_get_panels     (void);
-void     cc_panel_loader_add_option_groups (GOptionContext  *context,
-                                            GVariantBuilder *builder);
 CcPanel *cc_panel_loader_load_by_name   (CcShell       *shell,
                                          const char    *name,
                                          GVariant      *parameters);