SW: drop deprecated cpufrequtils (+ drop scripts/GRMLBASE/36-cpufrequtils)

The cpufreq drivers are autoloaded and the powersave/ondemand driver
is mature enough. The linux-cpupower tools provide the binaries
as replacement for what cpufrequtils provided so far and we ship
them (with GRML_FULL) already.

Also see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=877016

Thanks: Michael Biebl for the pointer
Closes: https://github.com/grml/grml/issues/51