lib/mathn.rb: remove built-in methods

* lib/mathn.rb (Fixnum#**, Bignum#**, Float#**, Rational#**):
  remove as these are now built-in.  [ruby-core:63973] [Bug #10086]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47290 b2dd03c8-39d4-4d8f-98ff-823fe69b080e