tests/pm_pc8: remove compare_registers subtest

tests/pm_pc8: remove compare_registers subtest

It's not executed by default, and it's completely relying on Haswell
registers and on internal knowledge of how the Kernel is supposed to
work. Since we plan to test generic runtime PM on all supported
platforms, maintaining this test so it works on all those platforms
will be a pain. We already have some ideas on how to verify registers
that must stay at specific values from inside the Kernel, so let's
kill this test and wait until the proper Kernel code gets merged.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>