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

`Devise::RegistrationsController#create` sets `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", e.g. when attempting to register a new user.