drop `-Dbpfs=userhacks`

Once we get !47 (merged) merged, I wonder if we should not drop entirely the option to install userhacks.

I would be surprised to see that anybody needs to install all of them at once, when they should really only be installed on a per-case basis.

And given that now cargo compiles everything, the process would be:

• install udev-hid-bpf if not installed already
• hack in userhack
• ninja
• udev-hid-bpf install builddir/src/bpf/userhacks/my-awesome_bpf.bpf.o
• rince wash repeat