[ruby/irb] Moved development dependencies to Gemfile

As expressions in a gemspec file are evaluated at the build time,
but not the run time, the conditional in the gemspec will not work
as intended.

ruby/irb@42f364ea23

Author: nobu

lib/irb/irb.gemspec

@@ -37,9 +37,4 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = Gem::Requirement.new(">= 2.5")
 
   spec.add_dependency "reline", ">= 0.1.5"
-  spec.add_development_dependency "bundler"
-  is_unix = RUBY_PLATFORM =~ /(aix|darwin|linux|(net|free|open)bsd|cygwin|solaris|irix|hpux)/i
-  spec.add_development_dependency 'vterm', '>= 0.0.5' if is_unix && ENV['WITH_VTERM']
-  spec.add_development_dependency 'yamatanooroti', '>= 0.0.6'
-  spec.add_development_dependency "rake"
 end