Drop unused variable from deep.c

[r-barnes]

Causes -Wunused-but-set-variable to pass

[brandon-rhodes]

@r-barnes — Could you outline your situation; are you working with tools that set that -W flag unilaterally? If so, then we might want to pivot the XEphem build to do the check itself, so that we don't break it again in the future without knowing it.

[r-barnes]

@brandon-rhodes - I am working in a large codebase with a custom build system that applies -Wunused-but-set-variable to allow our code as well as to third-party code.

I don't have numbers handy (though I could generate them), but enabling -Wunused-but-set-variable has helped us identify a lot of dead code and more than a few bugs. If you're interested in enabling the flag, that'd be swell. I'm otherwise just passing fixes upstream as I go.