Remove importmap:pins rake task in favor of bin/importmap json CLI (#43)

This rake task was added in f42f53e
before the `bin/importmap` CLI existed. Its function is identical to
the `bin/importmap json` command, though, so let's remove it to
consolidate around the `bin/importmap` CLI (which also hosts the `pin`
and `unpin` commands).

Author: rmacklin

lib/tasks/importmap_tasks.rake

@@ -3,10 +3,4 @@ namespace :importmap do
   task :install do
     system "#{RbConfig.ruby} ./bin/rails app:template LOCATION=#{File.expand_path("../install/install.rb",  __dir__)}"
   end
-
-  desc "Show the importmap"
-  task :pins do
-    require Rails.root.join("config/environment")
-    puts Rails.application.importmap.to_json(resolver: ActionController::Base.helpers)
-  end
 end