Remove journal_user definition from migration

Creating activity entries just results in noise and is not being used
at the moment as using update_all bypasses Rails hooks.

db/migrate/20240116165933_add_derived_done_ratio_to_work_packages.rb

@@ -57,9 +57,4 @@ def rollback_from_derived_done_ratio
   def non_leaf_work_packages
     WorkPackage.where.not(id: WorkPackage.leaves)
   end
-
-  # Sets the system user as the author of the Journal
-  # entries generated by this migration. Creates the system
-  # user if it doesn't already exist.
-  def journal_user = User.system
 end