Remove the ability to set lot numbers from the bulk add lots page

[iragm]

This caused some odd issues with adding rows. IMHO there's never (?) a reason to set lot numbers and this can be safely removed.

Admins can still set lot numbers one at a time on the lot management page, although no one seems to use this.

[iragm]

Just discovered that if you change a lot's number, the HTMX call to reload the page raises 404 because the lot number is part of the URL.

And custom lot numbers can be too long and get cut off, although this is hacked around for the moment and is unlikely to happen anyway.

And there's #263.

And there's one admin who manually set their numbers because they didn't know it could be automatic.

These are adding up, and it seems like the right course of action is to remove the ability to set custom lot numbers. I need to audit things to make sure no one is actually using this, but to my knowledge no one is.