Remove unnecessary require statement

In Ruby 3.2, the Set class is built-in and does not require a require
"set" statement. It's automatically loaded when you use the Set
constant or call Enumerable#to_set.

Author: sferik

lib/openai.rb

@@ -11,7 +11,6 @@
 require "pathname"
 require "rbconfig"
 require "securerandom"
-require "set"
 require "stringio"
 require "time"
 require "uri"