[Ruby 3.0 support] Don't warn when uninitialized instance variable is accessed

[andrykonchin]

The PR fixes the following item:

[easy, cleanup] Accessing an uninitialized instance variable no longer emits a
warning in verbose mode. [Feature #17055]

Related issue - #2453

Changes:

• removed the warning emitting
• removed a comment that mentioned this behavior

[eregon]

I haven't added any unit test in the RubySpec suite because it IMHO doesn't make sense to test absence of a feature. I tested it manually:

I hate to disagree but I think we should add one (unless there is already one).
The reason is then ruby/spec specifically tests that in Ruby 3+ it's OK to use uninitialized ivars, it won't warn (or it's a Ruby impl bug), and also this makes it easier for other Ruby implementations to notice the change.

[eregon]

Looks great, thanks!