[tx] revert missing cid fail to warn

[kaydeearts]

Description

In #1628 , we made some commits that would deprecate from FAIL to WARN when a glyph did not have a CID defined if it was a CID-keyed UFO. Zachary found some issues with this with some further testing. We'll do this in the near future, just wanted to make a release without this for now.

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[skef]

lgtm