Remove config/spring.rb.

We no longer use Spring (https://github.com/rails/spring).

config/spring.rb

@@ -1,10 +0,0 @@
-# This file configures Spring, the Rails application preloader.
-#
-# @see https://github.com/rails/spring
-
-%w[
-  .ruby-version
-  .rbenv-vars
-  tmp/restart.txt
-  tmp/caching-dev.txt
-].each { |path| Spring.watch(path) }