API Removed "PastMember" cookie and template getter

The functionality is easy to replicate in custom controllers,
and is too rarely used to be placed in core.

This also removes the `Member::is_repeat_member()` getter
and the `PastMember`/`IsRepeatMember` template globals.

See https://groups.google.com/forum/#!topic/silverstripe-dev/b8K3wU64TXg
Merge pull request #2465 from chillu/pulls/remove-pastmember

API Removed "PastMember" cookie and template getter