Remove a redundant Symbol#to_s

Follow up #11791.

RuboCop runtime supports only Ruby >= 2.7, so it removes a redundant `Symbol#to_s`.

Author: koic

lib/rubocop/cop/metrics/utils/abc_size_calculator.rb

@@ -117,8 +117,7 @@ def simple_assignment?(node)
           end
 
           def capturing_variable?(name)
-            # TODO: Remove `Symbol#to_s` after supporting only Ruby >= 2.7.
-            name && !name.to_s.start_with?('_')
+            name && !name.start_with?('_')
           end
 
           def branch?(node)