capistrano: simplify unicorn:terminate task.

Use the builtin task provided by capistrano3-unicorn. Ours failed with
invalid pidfile errors in some cases.

lib/capistrano/tasks/app.rake

@@ -9,7 +9,7 @@ namespace :app do
       # do nothing
     else
       invoke "web:disable"
-      invoke "unicorn:terminate"
+      invoke "unicorn:stop"
     end
   end
 

lib/capistrano/tasks/unicorn.rake

@@ -1,25 +0,0 @@
-# https://bogomips.org/unicorn/SIGNALS.html
-namespace :unicorn do
-  desc "Terminate unicorn processes (blocks until complete)"
-  task :terminate do
-    on roles(:app) do
-      within current_path do
-        kill_unicorn("SIGQUIT")
-        execute :sleep, 10
-        kill_unicorn("SIGTERM")
-        execute :sleep, 2
-        kill_unicorn("SIGKILL")
-      end
-    end
-  end
-
-  def unicorn_running?
-    test("[ -f #{fetch(:unicorn_pid)} ] && pkill --count --pidfile #{fetch(:unicorn_pid)}")
-  end
-
-  def kill_unicorn(signal)
-    if unicorn_running?
-      execute :pkill, "--signal #{signal}", "--pidfile #{fetch(:unicorn_pid)}"
-    end
-  end
-end