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.lock

@@ -83,8 +83,6 @@ GEM
       zeitwerk (~> 2.3)
     addressable (2.8.0)
       public_suffix (>= 2.0.2, < 5.0)
-    airbrussh (1.4.0)
-      sshkit (>= 1.6.1, != 1.7.0)
     ansi (1.5.0)
     ast (2.4.2)
     aws-eventstream (1.2.0)
@@ -114,23 +112,6 @@ GEM
       msgpack (~> 1.0)
     builder (3.2.4)
     byebug (11.1.3)
-    capistrano (3.16.0)
-      airbrussh (>= 1.0.0)
-      i18n
-      rake (>= 10.0.0)
-      sshkit (>= 1.9.0)
-    capistrano-bundler (2.0.1)
-      capistrano (~> 3.1)
-    capistrano-deploytags (1.0.7)
-      capistrano (>= 3.7.0)
-    capistrano-rails (1.6.1)
-      capistrano (~> 3.1)
-      capistrano-bundler (>= 1.1, < 3)
-    capistrano-rbenv (2.2.0)
-      capistrano (~> 3.1)
-      sshkit (~> 1.3)
-    capistrano3-unicorn (0.2.1)
-      capistrano (~> 3.1, >= 3.1.0)
     capybara (3.35.3)
       addressable
       mini_mime (>= 0.1.3)
@@ -294,8 +275,6 @@ GEM
     multi_json (1.15.0)
     multi_xml (0.6.0)
     multipart-post (2.1.1)
-    net-scp (3.0.0)
-      net-ssh (>= 2.6.5, < 7.0.0)
     net-sftp (3.0.0)
       net-ssh (>= 5.0.0, < 7.0.0)
     net-ssh (6.1.0)
@@ -461,9 +440,6 @@ GEM
       actionpack (>= 4.0)
       activesupport (>= 4.0)
       sprockets (>= 3.0.0)
-    sshkit (1.21.2)
-      net-scp (>= 1.1.2)
-      net-ssh (>= 2.8.0)
     stackprof (0.2.17)
     stripe (5.38.0)
     stripe-ruby-mock (3.0.1)
@@ -522,11 +498,6 @@ DEPENDENCIES
   binding_of_caller
   bootsnap
   builder
-  capistrano (~> 3.10)
-  capistrano-deploytags (~> 1.0.0)
-  capistrano-rails
-  capistrano-rbenv
-  capistrano3-unicorn
   capybara
   clockwork
   codecov