test_bug_reporter.rb: remove core

* test/-ext-/bug_reporter/test_bug_reporter.rb (test_bug_reporter_add):
  remove expected core file to get rid of false error in chkbuild.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43867 b2dd03c8-39d4-4d8f-98ff-823fe69b080e