rails: remove sprockets references.

Remove a dummy Sprockets config file needed to get `bin/derailed` to
run. This is no longer necessary since Rails 7.0 no longer depends on
Sprockets.

app/assets/config/manifest.js

@@ -1,8 +0,0 @@
-// This is the config file for Sprockets. Danbooru doesn't use Sprockets, but
-// we still need this file anyway because the derailed_benchmarks gem doesn't
-// work without it. derailed_benchmarks does `require "rails/all"`, which loads
-// Sprockets, which fails if this config file isn't present.
-//
-// https://github.com/rails/sprockets-rails/issues/444
-// https://github.com/zombocom/derailed_benchmarks
-{}