Remove ostruct from dev deps (#281)

Fixes #280 

It seems that it's no longer needed.

Author: pboling

Gemfile

@@ -6,14 +6,6 @@ git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
 gemspec
 
 group :development do
-  # This is for suppressing the following warning:
-  #
-  #   warning: ostruct was loaded from the standard library, but will
-  #   no longer be part of the default gems starting from Ruby 3.5.0.
-  #
-  # This should be part of "json". We can remove this when "json"
-  # depends on "ostruct" explicitly.
-  gem "ostruct"
   gem "rake"
   gem "rdoc"
 end