(maint) remove fact stringifying deprecation warning

Fact stringifying is on by default, but we're issuing a deprecation
warning on stringified facts. Since it looks like we'll have to
stringify facts for a while, this commit removes the deprecation
warning.

This is an update of GH-1658 that takes the comments into account.
Merge branch 'adrienthebo-maint-remove_fact_stringify_warning'

* adrienthebo-maint-remove_fact_stringify_warning:
  (maint) remove fact stringifying deprecation warning

closes #1687