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

[Drew Adams]

> > It's still a vacuous definition.  And any defvar tells the 
> > byte compiler that a variable has dynamic scope, no?
> 
> It also tells the user, so having them in the imenu doesn't seem
> superfluous to me (I'm talking again of lexically-scoped packages).

Submenu `Variables' should be for variable definitions, not vacuous defvars that
might be used to indicate something to the byte compiler.

As I said, if you want to also present those to the user, then let's put them in
a  separate submenu.  They amount to byte-compiler declarations.  Perhaps there
are other byte-compiler-related constructs that could also be added to the same
submenu.  Call it `Byte-Compiler' or `Declarations', perhaps.

But these are not variable definitions in the same sense as full defvars are.
Mixing them in with full definitions, in the same submenu, just amounts to
noise.  Separating them out in a separate menu would be fine.