Revert workaround for YARD

YARD 0.9.21 has been released.
https://rubygems.org/gems/yard/versions/0.9.21

This PR reverts the following workaround code for YARD.

- #7598
- #7558

Gemfile

@@ -5,23 +5,14 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'bump', require: false
-# Workaround for YARD 0.9.20 or lower.
-# Depends on `e2mmap` and `irb` until the release that includes
-# the following changes:
-# https://github.com/lsegal/yard/pull/1296
-gem 'e2mmap'
-gem 'irb', '1.0.0'
 gem 'pry'
 gem 'rake', '~> 12.0'
 gem 'rspec', '~> 3.7'
 gem 'rubocop-performance', '~> 1.5.0'
 gem 'rubocop-rspec', '~> 1.33.0'
 gem 'simplecov', '~> 0.10'
 gem 'test-queue'
-# Workaround for YARD 0.9.20 or lower.
-# It specifies `github` until the release that includes the following changes:
-# https://github.com/lsegal/yard/pull/1290
-gem 'yard', github: 'lsegal/yard', ref: '10a2e5b'
+gem 'yard', '~> 0.9'
 
 group :test do
   gem 'safe_yaml', require: false