Retire app-menu

For https://gitlab.gnome.org/GNOME/Initiatives/issues/4,
the application menu is being retired, with it's contents
being moved into the in-window primary window.

When removing a menu, the shortcuts for it's actions (if defined
in the menu) are also removed. The "Quit" shortcut is now
defined in src/gcal-window.c.

Related to https://gitlab.gnome.org/GNOME/gnome-calendar/issues/319

data/ui/menus.ui

@@ -1,28 +1,5 @@
 <?xml version="1.0"?>
 <interface>
-  <menu id="app-menu">
-    <section>
-      <item>
-        <attribute name="label" translatable="yes">_Calendars</attribute>
-        <attribute name="action">win.show-calendars</attribute>
-      </item>
-    </section>
-    <section>
-      <item>
-        <attribute name="label" translatable="yes">_Keyboard Shortcuts</attribute>
-        <attribute name="action">win.show-help-overlay</attribute>
-      </item>
-      <item>
-        <attribute name="label" translatable="yes">_About</attribute>
-        <attribute name="action">app.about</attribute>
-      </item>
-      <item>
-        <attribute name="label" translatable="yes">_Quit</attribute>
-        <attribute name="action">app.quit</attribute>
-        <attribute name="accel">&lt;Primary&gt;q</attribute>
-      </item>
-    </section>
-  </menu>
   <menu id="add-source-menu">
     <section>
       <item>