Remove for -f to set factor

It is only valid for default_engine and should be configured
through factor=nn.n

Change-Id: Ie517b4df0fb4eaa62914585355003ee94f4c45dd
Reviewed-on: http://review.couchbase.org/33080
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>