guix package: Remove dead code and unused variables.

Suggested by Alex Kost <alezost@gmail.com>.

* guix/scripts/package.scm (options->installable)[deduplicate]: Remove.
  [packages-to-upgrade]: Remove unused variable 'newest'.
  (roll-back): Remove unused variable 'manifest'.
  (guix-package): Remove unused variables 'guile-missing?', 'verbose?',
  and 'packages'.