Remove y Object extension in IRB

Fixes: #685

This feature can easily break how you use other gems like factory_bot or prawn.

#747 (comment)
> But I kind of think we should leave `psych/y` around. If people really want to use it they could require the file.

If you miss the function in Kernel, you can require it interactively or add it to `.irbrc`:
```ruby
require 'psych/y'
```

Author: runephilosof

lib/psych/core_ext.rb

@@ -14,10 +14,6 @@ def to_yaml options = {}
   end
 end
 
-if defined?(::IRB)
-  require_relative 'y'
-end
-
 # Up to Ruby 3.4, Set was a regular object and was dumped as such
 # by Pysch.
 # Starting from Ruby 3.5 it's a core class written in C, so we have to implement