use includes instead of joins at Issue#visible

"rake test" passes all test, but following test fails on command line.

<pre>
$ ruby test/unit/issue_test.rb -n test_visible_scope_for_member_with_groups_should_return_assigned_issues

  1) Failure:
IssueTest#test_visible_scope_for_member_with_groups_should_return_assigned_issues [test/unit/issue_test.rb:321]:
Failed assertion, no message given.
</pre>

git-svn-id: http://svn.redmine.org/redmine/trunk@13500 e93f8b46-1217-0410-a6f0-8f06a7374b81
remove unneeded "references" of joins

"references" is needed for includes, but not needed for joins.

git-svn-id: http://svn.redmine.org/redmine/trunk@13501 e93f8b46-1217-0410-a6f0-8f06a7374b81