CBD-926: Remove bundled libevent

Change-Id: I7a3ae649e36d557d74d106d9e021d785157903b8
Reviewed-on: http://review.couchbase.org/25363
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>