Fix redundant assignment of self.resource in Devise::RegistrationsController

Devise::RegistrationsController#create set self.resource (on line 13) to the return value of build_resource--which is nil, because build_resource sets self.resource to an actual resource object. This caused attempting to save the resource (two lines down, on line 15) to fail with "undefined method `save' on nil:NilClass" when attempting to register a new user.