Remove outdated ruby-debug requirement

[smasato]

This PR removes an outdated debugging requirement from our test helper and Gemfile. Because Our test matrix in .github/workflows/test.yaml only includes Ruby versions 2.6 and above.

Additionally, JRuby will never meet the condition RUBY_VERSION.to_f < 1.9.

❯ mise exec ruby@jruby-9.4.8.0 -- ruby -e 'puts "#{JRUBY_VERSION}, #{RUBY_VERSION}"'
9.4.8.0, 3.1.4
❯ mise exec ruby@jruby-9.3.15.0 -- ruby -e 'puts "#{JRUBY_VERSION}, #{RUBY_VERSION}"'
9.3.15.0, 2.6.8