Remove :emacs imenu module

Some of it was moved into core as reasonable defaults, but the rest of
it doesn't warrant a whole module dedicated to it.

init.example.el

@@ -59,7 +59,6 @@
        ;;+icons          ; colorful icons for dired-mode
         )
        electric          ; smarter, keyword-based electric-indent
-       imenu             ; an imenu sidebar and searchable code index
        vc                ; version-control and Emacs, sitting in a tree
 
        :term