Don't register taskable for stable functional test runs
Don't register taskable for stable functional test runs

FdbTaskable registration is not used anymore as new compaction
daemon uses its own taskable implementation.
Disabling this registration prevents random failures in
commit validations in master branch.

Change-Id: Id852fefe53d97a034ef442aaf77ee295d3616c81
Reviewed-on: http://review.couchbase.org/68757
Reviewed-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

couchbase/forestdb,wurikiji/forestdb,wurikiji/forestdb,couchbase/forestdb