Remove catalogue item start and end columns

Nothing in REMS sets the end time for a catalogue item (at least after #2283 is fixed), and we only use the start & end to compute whether the catalogue item is active. Instead, we could only use the catalogue item enabled status bit to decide activity.

Plan:
- [ ] Check if anyone uses catalogue item start and end. Probably only LBR could be using them.
- [ ] Remove code that reads start & end, remove code for `:active`
- [ ] Write a migration that disables any item with and end date in the past (NOTE: this would be an "impure" migration as items with an end date in the future lose that feature)