MB-100 Don't set ISASL_DB_CHECK_TIME env variable for memcached.

It's not used anyway.

Change-Id: I20275f617bfee05a5713737634c0dca437a23586
Reviewed-on: http://review.couchbase.org/29015
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>