CBD-926: Remove plaintext sasl db

Change-Id: Ibff6e18125462cc969e9304732c07d37cb2096b1
Reviewed-on: http://review.couchbase.org/25355
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>