Schedules no longer have a user

As announced in 2019, Schedules no longer need a user 🏃🏼‍♂️💨

Schedules created before then and not manually migrated have now had their build ownership user removed. Builds created from those schedules will no longer have a creator, which may affect trigger step permission, build.creator conditionals, and $BUILDKITE_BUILD_CREATOR environment variable checks.


Schedules created since the 2019 announcement are unaffected, as they never had a build ownership user.