Deprecate `tabindex` functions and their usages. Props joedolson. Fixes #2714.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5561 9866e705-20ec-0310-96e7-cbb4277adcfb
Missed removing a tabindex usage. See #2714.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5562 9866e705-20ec-0310-96e7-cbb4277adcfb