CBD-926: Remove doxygen support

Change-Id: I0f36eaa76721746ed967c882cfa2a62078d5d35f
Reviewed-on: http://review.couchbase.org/25366
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>