[bundler/bundler] Not sure how much this buys us, remove it

https://github.com/bundler/bundler/commit/f8584fb864

lib/bundler/templates/newgem/newgem.gemspec.tt

@@ -31,6 +31,4 @@ Gem::Specification.new do |spec|
 <%- if config[:ext] -%>
   spec.extensions    = ["ext/<%= config[:underscored_name] %>/extconf.rb"]
 <%- end -%>
-
-  spec.add_development_dependency "bundler", "~> <%= config[:bundler_version] %>"
 end