CBD-926: Remove configure test for dropping privileges

Change-Id: I64bc97ea0682323169136c18fb5e643a8f4b3a86
Reviewed-on: http://review.couchbase.org/25364
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>