[ruby/logger] Drop RDoc development dependency to avoid CI failures.

(https://github.com/ruby/logger/pull/82)

https://github.com/ruby/logger/commit/8915627234

lib/logger/logger.gemspec

@@ -23,5 +23,4 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", ">= 0"
   spec.add_development_dependency "rake", ">= 12.3.3"
   spec.add_development_dependency "test-unit"
-  spec.add_development_dependency "rdoc"
 end