Remove E016 for ADDED trips

[barbeau]

Summary:

We currently have E016 for ADDED trips:
https://github.com/CUTR-at-USF/gtfs-realtime-validator/blob/master/RULES.md#E016

ADDED has recently been marked as "unspecified" in GTFS-realtime, which means that the exact behavior isn't defined:
google/transit#230

We should probably remove this error, as some producers could consider this case valid, and instead add a warning that ADDED trips are unspecified and behavior of consumers isn't consistent.

Steps to reproduce:

Run the validator

Expected behavior:

Don't throw errors on unspecified behavior

Observed behavior:

ADDED is now unspecified in GTFS-RT, but we throw errors on it based on our past assumption of how it works (which, it turns out, others disagree with - google/transit#230).