Revert r46977.  [ruby-core:63857] [Bug #10071]

The removed comment is useful to understand the behind idea to
solve the year 2038 problem.

The broken link is replaced to refer the internet archive.
https://github.com/ruby/ruby/commit/0eed69757dd2bc0eb460a5dc38e377ac8e2cbfd9#commitcomment-7166665



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47179 b2dd03c8-39d4-4d8f-98ff-823fe69b080e