because they're provided as "default gem" since Ruby 2.6.

We provide default gems as gem ports rather than a part of lang/ruby* port.
files/patch-tool_rbinstall.rb makes not to install such default gems
however hollow wrapper scripts bin/{irb,ri,rdoc} and their manpages are
still left.

This changeset intend to move out such wrapper scripts and manpages to
gem ports completely.