

Delete windows partition with your preferred tool and update-grub
should remove the item from boot menu. Then, depending on your partitioning schema, you can either create a new partition in the empty space and mount it however you like or expand your existing linux partition, but options there depend on how your partitioning has been originally built and if you can leverage things like LVM or ZFS when expanding the usable storage.
And, while pretty obvious, make sure to only delete the correct partition and all data stored on that will be lost, so make sure you don’t have anything important on windows side of things.
There’s still things like that on my workplace today. I think there’s some older, rarely used CNC with Win98 on the controller. We just keep spares around when they break, but that’s cheaper than replacing the whole machinery. Also there’s some XP stations running software for an industrial machine which would cost quarter of a million to replace. Some of those need access to network drives and such but they live in a strictly isolated VLAN.
And, as far as I’ve told at least, there was no option at any point to upgrade just the computers on those things. It’s always the whole assembly line or whatever they’re connected to. There’s not many companies willing to throw hundreds of thousands every 3-5 years to replace perfectly working equipment.