* todos.el: Fix byte-compiler warnings.

* todos.el: Fix byte-compiler warnings.
(todos-merge-category): Relocate let-bound variable.
(todos-toggle-item-highlighting): Restrict to todos-mode,
todos-archive-mode and todos-filtered-items-mode.
(todos-toggle-item-header, todos-diary-goto-entry)
(todos-insert-sort-button): Let-bind variables that were
mistakenly free.
(todos-convert-legacy-files): Just require todo-mode instead of
conditionalizing it.
(todos-modes-set-2): Check whether hl-line-range-function is void.