remove fts strings from cbgt's lean Cfg

My bad as a code reviewer for missing this layering violation, but the cbgt library should not have any mention of FTS or CBFT – as that's only one particular application that's using the cbgt library.

https://github.com/couchbase/cbgt/blob/master/cfg_metakv_lean.go#L40

Looks like this was introduced in commit 8a140fa3, 2017/09/11, so it might have already gone production already in customer deployments?  (Need to double check on that.)  If so, that'd be unfortunately, and we'd need to carefully have a backwards compatibility maneuver to handle the mess.