test_date.rb: nonsense assertion

* test/date/test_date.rb (TestDate#test__const): remove nonsense
  assertion.  nil.frozen? is not date's concern.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47515 b2dd03c8-39d4-4d8f-98ff-823fe69b080e