boot: stop making TPM PCR to measure kernel command line into configurable

boot: stop making TPM PCR to measure kernel command line into configurable

Everyone appears to use PCR 8 for this, hence I think it's safe to
hardcode that in systemd too.

It's also documented, like here:

https://www.gnu.org/software/grub/manual/grub/html_node/Measured-Boot.html

or here:

https://github.com/rhboot/shim/blob/main/README.tpm

(And the previous name was a bit confusing, since we don't actually just
measure one thing anymore, but mutliple things into multiple PCRs...)