Add a CI check for REMOVE BEFORE FLIGHT

[eggrobin]

We just merged this: https://github.com/unicode-org/unicodetools/blob/e9cb07502ef4fa730eefbf507272bf1264fc60f8/unicodetools/src/main/java/org/unicode/text/UCD/UCD.java#L1312-L1316

Might also do DO NOT SUBMIT which is standard Google practice.

(I wrote such a check a in another place year and a half ago, so this is easy.)