Remove duplication of plate creation date info in db

https://github.com/jdereus/labman/pull/412 introduced a new creation_timestamp column into the labman.plate table. Technically, this is a duplication of data: plate creation dates are already stored in the db as the run_date of the *process* that created the plate. If we ever have time, we may want to clean this up.

Note this is the same sort of issue as that described in https://github.com/jdereus/labman/issues/220 : convenience fields in db cause data duplication and thus the potential for multiple (different) values where there should only be one.