Remove redundant, broken RedirectBackError rescue

Replacement of `redirect :back` with `redirect_back` with an option
for `fallback_location:` meant this rescue was already unnecessary in
Rails 5.0.

app/controllers/authentications_controller.rb

@@ -1,10 +1,6 @@
 class AuthenticationsController < ApplicationController
   skip_before_action :authenticate_user!
 
-  rescue_from ActionController::RedirectBackError do |exception|
-    redirect_to edit_user_registration_path
-  end
-
   def create
     omniauth = request.env['omniauth.auth']
     provider, uid  = omniauth.values_at('provider', 'uid')