Fix unused variable warnings.

In b0f96cb0a60caaccc1c6ea5f981ad8dca09d025c the variable `DoUpdate` was
renamed to `Update`¹, re-introducing warnings that were fixed in #173.

This change removes said warnings.

Tested on the GUI; I can tick the update box just fine, and indeed just
updated my version of DeadlyReentry. :)

Attn: @AlexanderDzhoganov in case I've screwed anything up, but I'm
pretty sure I'm good. :)

¹ Git bisect is *seriously* the best thing ever.
Merge pull request #277 from pjf/fix_warnings

Fix unused variable warnings.