This change removes TrialDivision#cache, TrialDivision#primes, and
TrialDivision#primes_so_far.

TrialDivision class is undocumented officially, so this class is used
only internally.  Yugui san moved prime library from mathn to prime in
2008, and then she might forget to delete these methods.

A patch from @shio-phys.  https://github.com/ruby/prime/pull/4

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63737 b2dd03c8-39d4-4d8f-98ff-823fe69b080e