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).

Gemfile

@@ -12,9 +12,6 @@ gem 'ruby-vips'
 gem 'net-sftp'
 gem 'diff-lcs', :require => "diff/lcs/array"
 gem 'bcrypt', :require => "bcrypt"
-gem 'capistrano', '~> 3.10'
-gem 'capistrano-rails'
-gem 'capistrano-rbenv'
 gem 'rubyzip', :require => "zip"
 gem 'stripe'
 gem 'aws-sdk-sqs', '~> 1'
@@ -59,12 +56,10 @@ gem "parallel"
 
 group :production do
   gem 'unicorn', :platforms => :ruby
-  gem 'capistrano3-unicorn'
 end
 
 group :production do
   gem 'unicorn-worker-killer'
-  gem 'capistrano-deploytags', '~> 1.0.0', require: false
 end
 
 group :development do