How to safely remove fields from create content - project
Normally I would suggest just hiding the unwanted field via a hook_form_alter, either by setting $form['unwanted_field']['#access'] = FALSE, or by using the hide() on that element. If the field is a required field, you'll want to alter that property as well, or set a value before hiding it or setting '#access' to false.

if you want to actually delete the field, you can use drush to run field_delete_field(). For example, drush eval 'field_delete_field("unwanted_field")' should work (untested).