lib/prime.rb: remove unused methods

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

lib/prime.rb

@@ -391,13 +391,6 @@ class Prime
       @ulticheck_next_squared = 121   # @primes[@ulticheck_index + 1] ** 2
     end
 
-    # Returns the cached prime numbers.
-    def cache
-      @primes
-    end
-    alias primes cache
-    alias primes_so_far cache
-
     # Returns the +index+th prime number.
     #
     # +index+ is a 0-based index.