measure: drop incomplete support for PCRs != 11

measure: drop incomplete support for PCRs != 11

At this point we have a clearer model:

* systemd-measure should be used for measuring UKIs on vendor build
  systems, i.e. only cover stuff predictable by the OS vendor, and
  identical on all systems. And that is pretty much only PCR 11.

* systemd-pcrlock should cover the other PCRs, which carry inherently
  local information, and can only be predicted locally and not already
  on vendor build systems.

Because of that, let's not bother with any PCRs except for 11 in
systemd-measure. This was added at a time where systemd-pcrlock didn't
exist yet, and hence it wasn't clear how this will play out in the end.