Remove Capistrano.

No longer used now that we use Kubernetes to deploy the site instead of
Capistrano.

If you run your own installation of Danbooru, and you used Capistrano to
deploy your site, it is recommended that you switch to either the Docker
Compose file (for personal installs), the Procfile (for non-Dockerized,
development environments), or Kubernetes (for production environments;
see https://github.com/danbooru/danbooru-infrastructure/tree/master/k8s
for Danbooru's production configuration).

bin/cap

@@ -1,29 +0,0 @@
-#!/usr/bin/env ruby
-# frozen_string_literal: true
-
-#
-# This file was generated by Bundler.
-#
-# The application 'cap' is installed as part of a gem, and
-# this file is here to facilitate running it.
-#
-
-require "pathname"
-ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
-
-bundle_binstub = File.expand_path("../bundle", __FILE__)
-
-if File.file?(bundle_binstub)
-  if File.read(bundle_binstub, 300) =~ /This file was generated by Bundler/
-    load(bundle_binstub)
-  else
-    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
-Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
-  end
-end
-
-require "rubygems"
-require "bundler/setup"
-
-load Gem.bin_path("capistrano", "cap")