bug#8638: 24.0.50; Imenu should not include vacuous defvars

[Drew Adams]

> > it is a judgment call based on expectations of what most 
> > users will want/expect.
> 
> Not users. Developers. And developers are usually interested in a
> package's variables. More so in a lexical-binding context, where the
> fact that the defvar exists can (and does) alter semantics.

Users includes users who develop, and more.  Imenu is for all Emacs users,
including developers.  And the question is not whether developers are interested
in a package's variables.  It is whether the Variables submenu should include
entries for vacuous defvars.