instancepool: Remove older C prototypes

Unfortunately we overlooked this in the previous commit

Change-Id: I53a55b1db4536e61a67c496f04d024a6cc738714
Reviewed-on: http://review.couchbase.org/39954
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>