Drop vocabulary/field selection page

There is a general settings page currently with two select fields to select the vocabulary and the taxonomy reference field to be used.

As the comment already states, I think we should drop that, I'm assuming that changing these values will result in weird errors and I don't really see a use case for them.

/**
 * Menu callback: Simplenews admin settings - General.
 *
 * @todo Delete this for if there is not more use for it.
 */
function simplenews_admin_settings($form, &$form_state) {

Patch follows.