[gtkmm-documentation] Remove the PlacesSidebar example

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 8c715d6b186f820940bc2abd190cb5509741ba61
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Fri Jan 12 16:49:21 2018 +0100

    Remove the PlacesSidebar example
    
    Gtk::PlacesSidebar has been removed. GdkPlacesSidebar is no longer
    public API.

 examples/Makefile.am                         |    6 -
 examples/book/placessidebar/examplewindow.cc |  155 --------------------------
 examples/book/placessidebar/examplewindow.h  |   54 ---------
 examples/book/placessidebar/main.cc          |   29 -----
 4 files changed, 0 insertions(+), 244 deletions(-)
---
diff --git a/examples/Makefile.am b/examples/Makefile.am
index f58b10a..c3f0df5 100644
--- a/examples/Makefile.am
+++ b/examples/Makefile.am
@@ -85,7 +85,6 @@ check_PROGRAMS =                                      \
        book/multithread/example                        \
        book/notebook/example                           \
        book/paned/example                              \
-       book/placessidebar/example                      \
        book/popover/example                    \
        book/printing/advanced/example                  \
        book/printing/simple/example                    \
@@ -548,11 +547,6 @@ book_paned_example_SOURCES =               \
        book/paned/messagetext.cc       \
        book/paned/messagetext.h
 
-book_placessidebar_example_SOURCES =           \
-       book/placessidebar/examplewindow.cc             \
-       book/placessidebar/examplewindow.h              \
-       book/placessidebar/main.cc
-
 book_popover_example_SOURCES =         \
        book/popover/examplewindow.cc           \
        book/popover/examplewindow.h            \