I'm looking at solving the same problem today and since this is not answered, giving it a go =)
Models
Devise generates a User model if you installed by default.
Remove the lines under devise. This is how mine looks like.
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
In attr_accessible, you may remove email, :password, password_confirmation and remember_me if you no longer need them.
Views
A default Devise install doesn't generate views in your app folder. If you generated overriding views for Devise, you may remove them by running rails destroy devise:views (Rails 3).
Generally, all the views are stored in app/views/devise.
Controllers
By default, Devise doesn't generate any controllers too. If you did any overrides, they are most likely known as registrations_controller. Search your project for controllers that inherit Devise::RegistrationsController class.
Also, if you followed Devise's wiki and monkey-ed around to add redirect methods etc, look out for methods such as after_sign_in_path_for, store_location etc that are for redirecting users.
Migrations
If you installed Devise via its generators, look out for a migration create_users. If you don't need it anymore, use drop_table :users in a migration to get rid of it.
I'll assume most people would want to keep their User model. If you're using Devise < 2.0, the migrations are done by helpers. Once you remove Devise from the Gemfile, Rails will not understand the helpers below and throw errors, for instance, when you're trying to rerun these migrations on another box. These helpers are:
t.database_authenticatable
t.recoverable
t.rememberable
t.trackable
t.encryptable
t.confirmable
t.lockable
t.token_authenticatable # => becomes t.string :authentication_token
For the exact columns, the below is reference to the columns generated by Devise.
https://github.com/plataformatec/devise/wiki/How-To:-Upgrade-to-Devise-2.0-migration-schema-style
The guide above lists the fields generated by Devise using the helpers. You should be able to look through the list and your model (e.g. calling User in console), generate a migration that removes those columns.
BUT...
It's a little unfortunate that for consistency, we have to convert the migration to not use helpers using the guide above then generate a migration to remove them. This is for migration history consistency, otherwise anyone running the migrations might try to call the non-existent helpers. Also, your migration to remove the fields will also expect the fields to be present.
Alternatively, it might be a good time to squash the migrations and rely on schema.rb / structure.sql for the schema's to-date state. Even after deleting migrations, you can always recreate your development DB anytime using rake db:schema:load.
Initializers and Locale
Remove devise.rb in config/initializers and devise.en.yml in config/locales.
Routes
Remove any devise_for lines. These will raise errors after the removal of the gem.
Gem File
Yaay. All dome, remove the line gem 'devise' from your gemfile.
