Remove obsolete -V short flag from --visibility option

The -V flag was registered on both --visibility and --verbose. Since
OptionParser resolves short-flag conflicts in favor of the later
declaration, --verbose owned -V in practice, leaving the visibility
short flag dead. Drop it to clear the duplicate registration.

Author: claude

lib/rdoc/options.rb

@@ -895,7 +895,7 @@ def parse(argv)
 
       opt.separator nil
 
-      opt.on("--visibility=VISIBILITY", "-V", RDoc::VISIBILITIES + [:nodoc],
+      opt.on("--visibility=VISIBILITY", RDoc::VISIBILITIES + [:nodoc],
              "Minimum visibility to document a method.",
              "One of 'public', 'protected' (the default),",
              "'private' or 'nodoc' (show everything)") do |value|